According to BB-Justin:
(306633.924)
Experience modifies a players skills during the end of the game.
I don't think you can quantify the exact impact with numbers, you mostly just get a feel for these things over time.
When starting out it's not something I would prioritise, for example I'd rather have a 24 year old (with same skills / salary / transfer fee but less experience) than a 30 year old, because you get more years out of them before they start declining and need to be replaced. But older players are often cheaper so it may work out that you have a couple of experienced players on your team.