I have quite a few Japanese maple cultivars, most of them in big garden pots, and a lot bear flowers and then samarras.
On my bonsai, almost none of them flower, whatever their age.
But this year, a friend spotted some flowers I hadn't seen on this A. p. 'Phoenix' from air-layering, 25 cm tall (ten inches) :
But the flowers fell off after a couple of days.
On larger trees that are not regularly pruned, you can have fruit, but the chance the seedlings you can get are true to the cultivars are very slim. It's fun though because sometimes they just partially revert to the parent plant or hybridize, and you can find an interesting seedling sometimes: that's how new cultivars appear.