Gender reassignment treatments should be restricted by at least age. For the sake of the person's health it should wait till at least after they are done growing through puberty. For example the sake for their health if it's important enough for them then it's worth that much of a wait.

The second thing to consider is that at the same time as puberty, (and a few years after) people go through a wide shift of who they identifty themseves as. For their own well being I would say at least a few years after puberty and an adult on their own before being given the chance of gender reassignment. Because it's not easily reversible, if it's reversible at all.