The tricky thing
Is yesterday we were just children
Playing soldiers, just pretending
Dreaming dreams with happy endings
- "Eyes Open"
They've run into a whole new world now, but they did it together - James & Sirius & Remus & Peter & Lily.
They're adults now, but only by technicality - they're still children, still young and innocent and almost {but not quite} untouched. Only yesterday, they were just children, innocent and playful. They still are, but to the rest of the world, they're 'adults'.
{nothing's changed, though}
James still whispers tales of grandeur to Lily, nuzzling her neck in bed, telling her 'everything's gonna be alright, love' because he honestly thinks it will be, and she believes him with the innocence of a child.
Sirius can still truly laugh, throw his head back and laugh, he's not scarred too badly, but he's less of a child than the rest. He grew up around darkness, he grew up fast. But he's still has a bit of childish innocence in him. He is still a child.
Remus, Remus is hopeful. It's naive and stupid, but oh, he hopes and prays everything will be alright. It's naive, but it is what Remus hopes. No, even serious Remus is a child.
Peter, Peter is unmarred. He thinks if he whispers Lily&James' home, he'll spare them and kill Harry. He doesn't understand the cruelty and the extent of Voldemort's atrocities.
No, they were only children.
But then Lily and James died and Peter was framed Sirius and Remus was all alone.
They are not children any more.