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Reading the Caf, you'd think players who turn 30 suddenly start moving around in wheelchairs and are totally useless.
He's probably still got 4-5 seasons at the top level in him and has been one of the best CMs around for ages. I didn't think he'd ever leave Bayern, but if there's ever a chance to get him, I'd snap him up. Really classy player and would help us hugely.
Well a lot of them if not most start declining in form.
Humans are at their physical peak when they are 23-27 years old (take or give year). This physical peak usually coincides with a players best period. That is not to say those players become useless when they reach the 30 year mark but they do start to decline. Some players can decline very fast very sudden with others it can be a long gradual process. It is hard to say which one you'll be in for with a 30 year old player but what you do know is they won't be at their best anymore and they have no future resale value. Often the physical decline can also increase the likelihood of injuries.
Which makes investing a lot of money in these players quite risky. And when we are talking about investing money it isn't just the transfer price is also the wages.
If you compare that a player of 25 years old the wages are usually significantly lower. There is most likely a substantial resale value at the end of his contract (or the possibility for another couple of years without needing to pay a transfer fee). And you have more chance of having a high performant player because they are at the peak of their ability.
Looking at the investment for Schweinsteiger:
Transfer price + Wages/year * years in the contract + Resale value we won't have (opportunity cost)
I'll make an hyphotecial example for Schweini
£10m transfer fee + £10m/year * 4 years + £20m resale value = £70m investment to have in our midfield for the next 4 years and that isn't the schweinsteiger from 4 years ago but the 30+ year old one that is having injury problems and will lose speed and energy and decline in form every year a little more.
Compare that to signing somebody like Gündogan
£25m + £5m/year * 4 years = £45m investment That is for a player at the peak of his ability.
Summary:
No 30+ doesn't mean they are useless but they are on the decline and the high wages + lack of resale value often makes the investment for such a declining player quite high even if the involved transfer fee to bring the here in the first place is on the low side (imagine such transfers with high transfer fee and they definitely wouldn't be worth it unless the quality of the player is truly exceptional).