Do the people they pick to play Lily and James typically look too old to be 21?

Definitely. I think they were trying to emphasize that they were parents, though, because the actors are already older than book characters so I think they wanted that extra oomf to make sure it was known.

But yes, I imagine James so much more full of life, as well as Lily, too. But we can't always get what we want, yeah?

Definitely. I think they didn't realise how young Lily and James were meant to be when they first cast them. It's only revealed on their gravestone in the final book, as I remember. What I don't understand is why JKR didn't point it out.