Thunder Bolt
Silly old bat
they say that all your cells regenerate and are replaced over time, some faster than others, so that after 7-8 years you are completely renewed. like Trigger's Broom. however i suspect that may be bobbins.
i'd hazard a guess its LIFO from general metabolism as the newer fat is more accessible, not behind layers of older fat.
Once you get to 45 there are cells which don't renew hence the 45 eye, and you find that older people end up wearing glasses or having laser treatment.