Welcome!
The only way to do it is to change their year of birth, which you need to do by editing the value in players.dbf. Basically, you have to give them the wrong year of birth, so that the game calculates their age accordingly. A step-by-step would be as follows:
1. Download
DB Commander.
2. Open the players.dbf file from a roster save folder.
3. Find the player you're looking for. Searching via the NAME field is generally the quickest and easiest method.
4. Change their BIRTHDATE. It's in the format yyyymmdd (eg 19750329 = 29th March 1975), so it's a matter of changing the year represented in the first four digits.
5. Move to another cell in the database to save the changes, then close DB Commander (or at least use the logout option in the File menu to close the database) if you're done editing.
6. Re-load the roster in-game. When you start a Dynasty game, the new age will be automatically calculated.
Comprehensive guides to the game's DBF files can be found
here in our Wiki. Make sure that a DBF file isn't open in DB Commander while you're trying to load or save it in-game (if you've got both open at the same time and are alt-tabbing between them). Keep backups in case anything goes wrong!