I don't think there is any hard and fast rule to average age of people being taken on by CTC Wings. I'm 29 and will be 30 two weeks after I start CP57 but I've also heard of 19 year olds and every age in between being selected.
Selection is based on your ability to learn to become a Commercial Airline Pilot and at each Stage your performance against a standard, albeit very very high, not your age.
I would imagine though that CTC have to think very carefully about the upper age limits (of which there are none anymore) due to the employability of a Wings Cadet by their sponsoring airline. These airlines have to be confident that you will be employable for as long as possible but it's a fine balance between how long you will be able to be employed for and what life experience you will bring into this career.
