Age isn't as important as just improving the overall skill base. Age comes into factor because a 30 year old player with say, a 8k salary, will be cheaper than a 22 yo player with an 8k salary. They could be the same player but the older one will be cheaper because he isn't trainable and his skills will drop in a few seasons.
So don't just buy old players. Use salary as a guide, a rough guide, as to improving the skills. Salary is determined by the skills the player has respective to the position they play. To win in div 5 I think you need a starting 5 to average about a 7k salary, unless you have a ton of bots. More salary means less money you'll bring in, but it should definitely improve the quality of your players.