Lambo was ten when he fell in love with Tsuna, but there was a ten year difference in age, so he'd more or less given up hope on ever having Tsuna love him back, and spent his time becoming an upstanding member of mafia society to make his boss proud. Which was another problem. Tsuna was the boss and surrounded by amazing people all day every day. Lambo would be the first to admit that even though he tried his hardest, he was the weakest of all of the Vongola guardians, and also the most childish. For Lambo, when he complained about something, his problems always led back to the fact that he was young.
He'd basically grown up among people much older than him though, so he tended to dwell on it. Having to admit that even though he was one of them, he was still considered a child and left to do more childish work irked him. Especially because, being considered a child, Tsuna would never look at him in the way Lambo most wanted him to.
Or at least that is how Lambo felt until he turned fifteen and around then was forced to switch with his five year old self. For that past Tsuna, he was the same age so he wasn't a child to him. It really brought him back (both figuratively and literally) to those days when Tsuna wasn't as burdened by the sins of the Vongola. Sins he'd ordered and inherited. Tsuna was more fresh then, but the strength Lambo had come to associate with him wasn't there. Not without Reborn planting a bullet in his head anyway. Regardless, it was Tsuna, and for the five minutes he was there he was free to flirt with, talk to and admire Tsuna as much as he could.
Soon it wasn't enough for Lambo though. He always was a greedy individual wanting more than he could have, faster than he could get it. So when his younger self and he switched, and he found himself alone with Tsuna, he reacted instantly. He waltzed up to the young tenth, placed his palms on Tsuna's cheeks and sweetly kissed him. Just a small peck on the lips. That was enough. Lambo swore to himself that he would never do it again, or push harder and hurt the man/boy, or their boss subordinate relationship. The rest of Lambo's five minutes were spent standing there looking at eachother. Lambo was smiling lazily and Tsuna blushing.
When Lambo was back in his own time again, he was in his boss' office. Surveying the room, it looked like it had been turned into some sort of warzone, and the only other person in the room was Tsuna.
"What has my younger self been up to?!" he wondered aloud in a mix of terror, disgust and amazement. The adult Tsuna before him laughed kindly and approached him. Lambo's cheeks were grasped firmly is two large, warm palms and he was kissed by Tsuna.
Thinking back on it several days later, Lambo supposed he shoud have been more careful. Tsuna had been fifteen at the time, so it probably should have been obvious to Lambo that Tsuna would at least be able to remember, more than Lambo, because he didn't remember anything about his younger self having the Ten Year Bazooka. However, just then Lambo was incapable of any sort of thought. He was on cloud nine! Tsuna was kissing him! TSUNA was KISSING him!! TSUNA WAS KISSING HIM!!! It wasn't a very sweety kiss either. Far from it, in fact.
The end of the kiss came and went, and Lambo didn't remember many of the following events. He did reconsider the matter of his age though. Perhaps Lambo was the only one spending so much time fretting over it.
If you're old enough to kill be in the mafia as a more or less capable Vongola Guardian, you can certainly be old enough to kiss and love other people (and be loved in return) ! Or so decided Lambo, but don't quote him on that. Ten years older or not, Lambo is Lambo.
