Try this, i'm not sure if it works, i've thought about it but i'm currently testing some other stuff
Download this "customized" editor. Before installing, go in the "editor\format\database" folder inside and edit the "club.xml" file in notepad++ or something.
Search for the line "<flags id="field" value="Cnag" />" and a little below, there are the values permitted for using in the editor (by default you can't set the min. age to over 21). They are the lines "<integer id="min_value" value="16" />" and "<integer id="max_value" value="21" />".
You can really set the min to 14 and the max to 45, anything under that min. or over that max. will not work on the editor even if you set in that line, probably because it's related to somewhere else where it sets the min. and max. age for players, and i don't know where it changes and if it's possible.
Also, i would recommend doing the same thing in the "<flags id="field" value="Cmag" />" just above. (Max. Age)
Save the files and install the custom editor as described in the topic, then run the editor and set the age limits in the clubs you want, save the files and test ingame.
I think if you set a club to a minimum age of 30 for example, save, and run the game, it will probably work. Over 40 probably the AI will retire the players in a season or even less, and maybe other bugs. Also, i don't know if they will actually sign appropriately players. But this will probably work perfectly with greyed-out amateur clubs. Try just setting the minimum age and don't set a maximum age. Also, will be interesting to see how the youth intake day will work.
I don't expect this to work properly with over-40 teams, FM force players to retire older players quickly after you start a new game, even top players like Gianluigi Buffon or Cristiano Ronaldo.