frankly, both of these guys are both right and wrong.
In the game, there are tons of ways to develop players. That guy may have had success not loaning 19 year olds out, many other players likely have had success loaning 19 and even 18/17 year olds out. Some people loan 16 year olds out. It all will work to some extent.
One thing to consider is the player's skillset. If he requires mental growth, keeping him at home and using him however you'd like can be best. If he requires physical growth, he should get lots of games somewhere.
I've read on these forums that a reserve game is worth next to nothing for a player's development, so I avoid using my players in that squad.
If I had a large contingent of 17/18/19 year old wonderkids, I would do my best to play them as a group in league and association cups and other games when possible. Really satisfying to watch a squad full of 18 year olds win an FA cup

. I would also make them the 2nd or 3rd player at their position, so they will end up with 10+ games.
Thats my watermark. If I think a player will get 10+ games in the first team, I will keep him, if not, I tend to loan them out to teams that promise to play them a lot and have good facilities.
Lots of variations though. Im sure many folks have had lots of success allowing their best youngsters to play in the reserves regularly.
Quality tutoring is almost always essential though, so keep that in mind.