If a child enters the US with a tourist visa but overstays and doesn't leave until he is after 18 years old, can he later return to the US?

It depends on a number of factors such as in what status does he intend to return and whether he is subject to the penalties of unlawful presence bar.As a general rule,a child does not begin to accrue unlawful presence in the U.S until he reaches  18 years of age despite the authorized period of stay as stated on the I-94 having expired.

 If after turning 18years of age that child remains in the U.S for 180days after his 18th birthday,the day the child exit or leaves the U.S that child cannot come back to the U.S for 3 years. This is call the unlawful presence bar. If the period climbs up to 365 days or one year from when the child turns 18 years of age,the child will be barred from returning to the U.S for 10years.

So depending on how much time the child has accrued after turning 18 years of age,he may be subject to the 3 or 10 years unlawful presence bar.

The only way to overcome the unlawful presence bar before the expiratin of the 3/10 years bar  will be for the child to file a waiver application at the time of seeking a visa into the U.S.

 Please note that what triggers the bar is exit or departure from the U.S

 I hope the above helps!