For example, bitcoins that have been on wallet.dat since 2012 have a higher value than bitcoins since 2024?
2. I am also interested in if you transfer bitcoins 2012 year to another wallet, and the change returns to the old one, will they both become bitcoins in 2024?
1 bitcoin is always 1 bitcoin, no more no less. If you move your 1 bitcoin from your old wallet, assume it created in 2012, to your new wallet created in 2024, you will have less than 1 bitcoin because some satoshis will be spent to miners as transaction fee. Exception only if you luckily get its confirmation for zero fee like some mining pools do with their mined blocks.
But 1 old bitcoin and 1 new bitcoin has same value that only depends on Bitcoin price on the market, and not depend on its age, old or young.
If you want cool things, try this
Check if you have any rare satoshi
https://magiceden.io/ordinals/discover-raresats