The Pennsylvania UTMA (20 Pa. CSA §§ 5301-5321) provides that for the purposes of the UTMA, the age is 21. In most cases, then, you would be correct to open an account for the custodian if the beneficial owner is under the age of 21.
The exception, I think, would be if the property in the custodianship was transferred to the custodianship under section 5306 ("Other transfers by fiduciary") or section 5307 ("Transfer by obligor"). If the custodian tells you that the funds in the account aren't from those two sources, you'd be fine opening the account. (If the property was transferred under 5306 or 5307, the age for the custodian's ending the custodianship and surrendering the property is 18.)
Note: There is a provision in § 5321 that would even allow you to open a custodial account after the beneficiary has reached 21 years of age. The circumstances are limited, and you should contact legal counsel for guidance in such cases.
John S Burnett