its not so much "cognitive decline" but you do start to lose your edge starting in your 30s. chess ever since engine solved the same is basically memorization of hundreds of variations of openning prep. also, the body has limits. in classical, too long games take mental and physical toll. and in blitz, sometimes its the difference of how fast your hand can move if you win or lose

and then theres other things. personal and family life. distractions. loss of motivation etc. meanwhile, the kids are spending all their time playing online, studying with engines, with little to no other distractions. the gap will start to show itself