It depends if they are seed grown or grafted. Grafted trees can flower when they are just weeks old, since the wood is being taken from mature trees that are often ready to flower. It really depends on where the scion wood was harvested on the tree. Upper, vigorous stems have the best chance of flowering. Seed grown trees will vary in age on their first flowering depending on the spceies of Acer and cultural conditions. Perhaps Daniel will have some average flowering ages for us on seed grown trees?
For seed-grown maples, I'd think around 8-15 years from seed for most species. Longer, maybe 20 years or more, for some of the larger, slower growing species if in fairly shady conditions.
