If you're talking maturity, sometimes never. Sometimes during the preteen years, because they have a great mentor and spiritual guide who helps them understand why they're really here. It depends upon the individual spirit (soul, true self, immortal child of God within), the environment, upbringing, teachings, values and beliefs. For some, being "adult" means wisdom. I pray for wisdom for all.
If you're talking about losing the sense of wonder, sometimes never. Sometimes during the preteen years, because the young person experiences something traumatic and loses their way; they have the childlike innocence beaten out of them. For some, being "adult" means being reasonable. I pray for unreasonableness, and childlike wonder and awe for all.
Being young-at-heart and mature are not mutually exclusive qualities. It would be wonderful if all of these ancient beings who share this reality with me were to remember who they really are and eliminate the barriers of separateness. Restore the bonds of love.