There is no set age at which one should get baptized. Each student grows and matures at a different rate.
فَكُلُّ شَخْصٍ يَخْتَلِفُ عَنِ ٱلْآخَرِ ،‏ وَلَا يَتَقَدَّمُ ٱلْجَمِيعُ بِٱلسُّرْعَةِ نَفْسِهَا .‏