Depends entirely on size of pony, size of teen, and what teen wants to achieve with pony. If teen is extremely competitive in certain sports, it’s much harder to compete successfully on a pony mainly due to expectations that kids will move onto horses at a certain point. However, if they are lightweight, the pony is able to carry them comfortably, and their competitive aspirations are mainly in the ‘for fun’ category, then there is no reason to sell pony on just yet.
A sensible rider to pony weight ratio is 15%. A 14.2hh pony would on average be around 400kg, so you would be looking at a rider weight (dressed to ride, including tack) of around 60kg. If the pony is fit and sturdy, and usually this applies to heavy breeds, and the rider a well balanced and quiet rider you can get away with up to 20%, so rider dressed to ride and tack up to 80kg.
I see many small adult riders competing locally very successfully in dressage (both unaffiliated and BD) and unaffiliated showjumping on ponies. Yet it seems to be the done thing in the local pony club that every child as soon as they turn 13 acquires a horse on account of them outgrowing the perfectly good pony they were on. My DD competed in TREC, and had 2 ponies, (14.3hh and 13.3hh) she rode each one of them at a World Championship. The 14.3hh was a good size for her, the 13.3hh was too small really but was the pony we had at the time. He was a bit on the steady side too, which was the main issue rather than size. The people I bought my horse from had been having their 15 year old son ride her, but he was competing well at international level on ponies as a showjumper so was going to spend his last 2 years in ponies, despite being a tall lad capable of riding horses. Teens can ride ponies, and do well with them, for much longer than most people think.
If your child has definitely grown out of the pony size-wise (my DS suddenly got up one morning and was 6ft tall, out of nowhere! Although he was well within the weight limits for the pony, as he only weighed the same amount as a ball of cotton wool) then really it is for the best all round if you accept the pony is no longer the right one for the teen, and find them a new home and a new horse for the teen. It will get really frustrating otherwise. Our 13.3hh was much relieved when, once he had finished his work for teen DD, he was loaned to a home with 2 much younger children.