I'm afraid I don't know why or when children started to form part of the bridal party, but I'm quite sure that bridesmaids do need to be older, in order to carry out their duties properly. When Prince Andrew married Sarah Ferguson, as sweet as the bridesmaids may well have looked, they were useless at arranging the bride's dress. On the two occassions (on entrance and at the exit) the bride's train needed adjusting as it was veering towards the side: in the first, the dress designer ran across the aisle to sort it out and in the second, Sarah had to ask a Westminster Abbey official to correct the problem. Both occassions should have been dealt with effortlessly by the bridesmaids had they been old enough to know or been told what to do.
I myself have the dubious pleasure of having my two young neices act as flower girls at my forthcoming civil partnership, though I have no idea exactly what I'm expecting them to do apart from look vaguely angelic! We have asked guests (most of whom have children) not to bring their kids because we'd end up with more children than adults!