A/N: This timeline is still in progress, but it's a rough idea of how and when things happened in my universe, and a rough idea of ages at present.
200 years ago: Legends Arceus takes place
100 years ago: The (currently) last war of the pokémon world occurs.
85 years ago: Greevil is born.
80 years ago: Evice and Eagun are born.
75 years ago: Beluh is born
67 years ago: Professor Rowan is born
64 years ago: Professor Oak is born
60 years ago: Hala is born
50 years ago: Giovanni, Archie, Maxie, Faba, and Ardos are born.
48 years ago: Mohn is born.
45 years ago: Andrea and Lusamine are born.
44 years ago: Bruce is born.
40: Eldes, Ash's father, Grace, and Grace's husband are born.
38 years ago: Delia Ketchum is born.
37 years ago: Professors Elm, Sada, and Turo are born
35 years ago: Team Zenith is formed with Archie and Maxie as founding members. Greevil founds Cipher with the riches he gained from the mines of the Under. Research into the Shadowfication Process begins.
34 years ago: Giovanni starts his pokémon journey.
33 years ago: Cyrus is born. Giovanni gives Andrea her first pokémon, a Meowth she dubs Meowy.
30 years ago: Andrea and Bruce meet for the first time at Professor Oak's laboratory as a starting trainer and apprentice researcher respectively. Wes, Rui, Professor Kukui, Professor Burnett, and Zinnia are born. Lusamine's father founds the Aether Foundation.
28 years ago: People start calling Andrea the Scratch Cat Girl because of her skill as a battler despite her team only consisting of Meowth.
26 years ago: Red, Blue, Green, Brendan, and May are born. Lusamine meets Mohn for the first time working as one of her father's scientists.
25 years ago: Jessie, James, Wally, and N are born. Team Rocket starts as a small-time gang run by Giovanni. Team Zenith falls apart due to conflicting goals among its members. After leaving Maxie and Archie begin the work towards creating Team Magma and Aqua respectively. Wes is taken in by a newly forming Team Snagem. Ghetsis starts his long-term plans for Team Plasma, including grooming N into a suitable puppet king.
24 years ago: Bruce Ray earns Official Professor's Aide status interning under Professor Oak. Mohn becomes an officially recognized pokémon professor.
23 years ago: Ethan, Kris, Silver, Lyra, Barry, Dawn, and Lucas are born.
22 years ago: Leon and Sonia are born.
21 years ago: Michael, Hilbert, Hilda, Bianca, and Cheren are born.
20 years ago: Ash, Gary, Serena, Goh, Chloe, Hugh, and Emma are born. Andrea and Bruce get married.
19 years ago: Calem, Shauna, Tierno, Trevor, Korrina, Nate, Rosa, Curtis, and Yancy are born.
18 years ago: Chase, Elaine, and Trace are born. Construction of Aether Paradise begins. Ash's father leaves Pallet Town to earn money to provide for his family and disappears.
17 years ago: Gladion is born. The Pichu that will one day become Ash's Pikachu hatches from his egg and finding himself alone is taken in by a tribe of Kangaskhan.
16 years ago: Max, Sun, Lillie, and Hau are born. Ash meets Serena and Goh at Professor Oak's Summer Camp becoming fast friends. They all see Mew for the first time, Goh becoming determined to someday catch it. The trio promise to start their journey together. Cyrus becomes driven to create a world without spirit, using the resources of Team Galactic to further those goals. The Pichu that would one day be Ash's Pikachu evolves into a Pikachu.
15 years ago: Grace's husband dies causing her to become obsessed with keeping her family close. Cipher begins their covert attempts to take over Orre. Nate's mother retires from being a pokémon nurse to be a full-time mother.
14 years ago: Hop, Gloria, Victor, Bede, Marnie, and Arven are born.
13 years ago: Cipher supplies Team Snagem with the Snag Machine. Nemona is born.
12 years ago: Sun starts training with Giovanni. Bruce gets transferred out of Kanto. Bede's parents go broke and leave him at an orphanage. Penny is born.
11 years ago: Leon starts his journeys in Galar with Sonia by his side. Grace and Serena have a falling out about Serena starting her journey at 10 in Kanto with Ash and Goh. Grace refuses to allow it, but her daughter promises herself that she'll do it anyway. Calem supports his sister which causes a rift between him and his mother. Professor Oak catches the Pikachu he will one day give Ash. Juliana, Florian, and Hugh's little sister are born.
10 years ago: The events of Pokémon Red/Blue, Ruby/Sapphire, and Colosseum start. Ash, partially motivated by his admiration for Red, starts his journey at the legal limit of ten and has to be joined by Serena and Goh along the way despite their wishes to start together. Gary starts at the same time because he refuses to be out done by Ash. The Galar Championship Cup happens, and Leon promises Hop he'll become Champion, keeping his word. The Holon Mew Expedition begins, leading to the creation of Delta Pokémon.
9 Years ago: Jovi is born.
Roughly 9 years ago: Ash begins traveling the Orange Islands.
Roughly 8.5 years ago: Ash becomes champion of the Orange League, begins traveling through Johto.
8 years ago: The Pokémon Tower is torn down so the radio tower can be built. Andrea Ray begins her time as a DJ. Michael starts doing the battle simulations at the Krane Research Lab. Hugh's little sister's Purrloin is stolen by the Shadow Triad. Hugh's grandfather passes away.
Roughly 7.5 years ago: Ash, Serena, and Goh begin traveling in Hoenn.
7 years ago: Pokémon Gold/Silver/Crystal and Diamond/Pearl/Platinum start. Michael's father is diagnosed with cancer.
Roughly 6.5 years ago: Ash begins the Kanto Battle Frontier. Serena challenges Kanto's contest circuit. Goh continues traveling with them.
6 years ago: Michael's father has an Eevee delivered to Professor Krane's Laboratory before passing away. Cipher restarts their operations in secret.
Roughly 6 years ago: Ash, Serena, and Goh begin traveling in Sinnoh.
5 years ago: Pokémon Black/White and XD: Gale of Darkness begin.
Roughly 5 years ago: Ash begins traveling in Unova.
4 years ago: Mohn's experiment goes awry leading to him going missing. Faba's manipulation to further break the Aether family begins. Calem begins hatching plans with his friends to get away from Grace, whether she likes it or not. Korrina becomes Shalour City's gym leader.
Roughly 3.5 years ago: Ash's Kalos journey begins.
3 years ago: Pokémon Black 2/White 2 and X/Y start.
Roughly 2.5 years ago: Ash becomes Kalos Conference Champion, later starts traveling through Alola.
2 years ago: Chase, Elaine, and Trip start their adventures in Kanto. Gladion steals one of the Type: Null and flees Aether Paradise. Through Faba's manipulations, Lusamine's addiction to Nihilego's toxins worsens and her verbal abuse turns physical for the first time. Bruce starts working in Galar under Professor Magnolia.
Roughly 2 years ago: Jessie, James, and Meowth decide to quit Team Rocket and keep Manalo Malasadas going in Alola. Ash starts working with Professor Cerise and joins the Coronation League.
1 year ago: Andrea, fed up with the scrutiny her son receives, decides to look into moving to Alola as she's always dreamed. Lusamine begins her frozen pokémon collection.
6 months ago: Bruce begins working in Holon to study Delta Species Pokémon.
3 months ago: Lillie flees Aether Paradise with Nebby and moves in with Professor Kukui.
Present: Sun and Andrea arrive in Alola.
1 year into the future: The events of Sword/Shield start.
1 and a half years into the future: Operation Star begins.
2 years into the future: The events of the Crown Tundra begin
3 years into the future: The events of scarlet and violet begin
A/N: This story is almost a year old, and to celebrate, I'm planning to do an AMA. Feel free to leave questions!