For some reason,
Jonas makes me think of a young, very sweet, kind-hearted, caring, and a bit shy boy. Not that I don't see it as a strong name, too - to me, those charasteristics are not signs of weakness. Anyways, overall it seems like a very "young" name to me.
You're not alone with the age thing - I absolutely adore "old", retro-chic names because I find many of them sophistocated and respectful.