I think you're confusing Mary Tudor, the daughter of Henry VIII, with Mary Tudor, the daughter of Henry VII en the sister of Henry VIII and Margaret Tudor. The Habsburgs, from the time that Maximilian was still the head of the house, were interested in a match between Charles and Mary (the latter, as in the elder of the two), who was a few (not many) years older. Anyway they would have been closer in age, than Mary and her OTL husband Louis XII of France.
However you're right that IOTL Mary Tudor (the younger) the daughter of Henry VIII was deemed too young for Charles V.