16 was considered the age of manhood, and long trousers were expected.. Younger teenagers (13-16 years) would wear shortish trousers just below knee length. Silly by today's standards, but that's the way things were in those days.
Right up to 1950s in many places.
Plus, baby boys wore dresses - not girly frocks but skirts - till they were potty trained. It was a practical matter to do with absence of reliable nappies. A old bloke told me he could remember his 'breeching party' ie when he got his first breeches.
Pickle, my eldest bro who is ten years older than me wore short trousers till he was 16 - it was the done thing as only rough boys wore jeans (!) and it was his school uniform in any case. Yes, and mum picked all his clothes.