It is not a big difference in training efficiency between an 18 and 19 year old but in the long run will that difference still make a big difference. If you compare with even older players it will be an even bigger difference in the long term.
If you keep training your 23 year old player he will surely be better than a cheap draftee for the coming 3 seasons or so but after that his advantage right now will be upbeaten and he will not really improve at all despite some low skills.
Your other players has ok skills for training but thing is that you can always get a similar 18 year old player for basically no money so why train an older player then? My player that I posted was at most marginally worse than your own players but as they were older it should be more propriate to train my player. I will soon kick my player as he got no bids for 1000. You will certainly get better players than mine for small money so you can opt for better than him.