No body knows for a fact how old Mario and Luigi are, but there are a few facts that we all know, yet they really don't help us solve much: 1) Yoshi is older than both the Mario Bros. However, there is no confirmation that the Yoshi from Yoshi's Story and Yoshi's Island is the same Yoshi that we know today. It could be a similar case to Donkey Kong. 2) Luigi is the younger sibling. Based on what we see in Super Mario Advance 3, Mario and Luigi are similar in age.
Who knows, maybe there is a whole different aging process in the Mario universe. That would explain so many things.