Mangosteen has a whole lot of variation even within the same line, so you will get a very small percentage that will fruit in 5 years, a small amount that will fruit at 6-7, a vast majority fruiting at 8, and some that can take a whole lot longer, as in 10+ years. This all in same soil, same conditions, same climate. This was reported to me by a farmer in OZ who planted over 1000 mangosteen trees. I've seen similar occurence happening in that 40 trees i planted.
BTW, it looks like you don't have the mangosteen caterpillar (yet). Nasty bugger introduced from Thailand. Goes mostly after mangosteen and G. hombroiana, but will also attack other Garcinias and Rheedias. Chews off all the new growth and flowers. You never ever see them. They must come out at 2-3 AM when deep in snooze land. Then at day break all disappear. Been spraying with BT, but difficult here with rain washing it off.