I feel sorry for him. In the normal scheme of things, no, a 21 year old wouldn't have anything in common with a 16 year old. But THIS 16 year old has precious little in common with his peers. It's not like many Olympic sports where competitors are often young. He won't be meeting many people his age with anything like his experience of life. I doubt many mothers of sweet 16s would be overly keen on their daughters dating someone who leads his life, and I doubt very much he is going to find much common ground with a kid with a Saturday job.
You are all talking about him like he spends all his time gaming in his bedroom, like many of his age. He is in full time employment. Many 21 year olds are still living on the bank of Mum and Dad. He is going to (sadly, in my view) have far more in common with his 21 year old than with some 16 year old who hasn't even left her home town.
She isn't dating some school kid. She is dating someone who has been working for 3 years, longer than many of her own alleged peers.
Those of you saying your 21 year olds wouldn't date a 16 year old, no, of course they wouldn't, because it's highly unlikely they know any 16 year olds in his circumstance.
It must be such a lonely place for him. Nothing whatsoever in common with your average 16 year old. Dates someone he is far more likely to resonate with and the internet tells him she's a dirty cow only after his potential money.