Philip, the ages come from the IGI "Extracted marriage record for locality listed in the record" - i.e. Parish Church of Kirk-Braddan (Isle of Man): LDS film no. 0106710.

Perhaps the transcriber misunderstood [over] age 21, as being 21 as you suspect, but these entries are usually recorded as "Full age" rather than "over 21". The IGI entry indicates that ages were given in the Braddan register at that time, but I don't know why some have been added to their records, and others haven't.

It could be checked at the MM or on LDS film.

Sue