In a perfect world I would like both brothers and sisters younger, but you know what, if a sister is brought up away from this culture, I mean I see families who are cultural Muslims letting their kids walk around in miniskirts and so forth and not putting Islam into you, they wear hijab due to culture, if sisters were brought up good, I dont think 14/15/16 is too young. Think about it, in the western world, girls at that age are ready for long term reletionships, the problem is that the men that are the same age as them are not, the guys end up cheating on them and so forth, though the girl is in 'love' with him he isn't as keen. (edit: I remembered, in college Muslims girls were into this too, wearing hijab and talking about Asif, and Ashraf and other muslim boys, I mean, think about it)
Thats why I think that age would be normal, since the western girls can fall in love at that age, then I dont see why the Islamic girls cant fall in love and marry at the age too.
As for the brothers, in the west unless you have a helpful family it is pretty difficult to marry and support a family. Brothers are asked to have a BA, then to be abit older than the sister, and also to be in a stable job. That's what I have come across. Unless you get a good Muslim family young brothers will find it hard to get married.
But yes, if the support was there I would like to think brothers should marry earlier, but then again, the brothers from among us who have been touched by western ideas would find it hard to be tied down at a younger age.