Overage players can still play in the u23s. I think the rule is a goalkeeper and three overage outfield players are allowed. Chelsea had 30 year old Danny Drinkwater playing last week while we had Pennington, who's 26.
I'm surprised we don't use the U23s more for players coming back from an injury if that's the case mate. Unless the idea is that the club wouldn't want to do it because it would have an impact on the progression of the players in the U23s squad.