With respect to Japanese maples specifically it is not a question of the age of the tree itself but rather of the age of the grafted stock. Even very small/young JM's can produce flowers and seeds, as the grafted portion is typically obtained from fairly mature stock.

FWIW, flowering and seed production of JM's can be very sporadic - some cultivars tend to flower very freely, others not so much. It can also be influenced by growing conditions.