The roulette wheel of genetics can be a harsh mistress. These two are twins, for example:
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But on top of that, Ari is a woman (genetics that can look good on a girl can look bad on a man) and she also seems to take care of herself.
Ari is butter cop while Sander's attempt at improving his nutrition was "eat an entire plate of plain arugula with every meal for a month" (which he failed after a week and a half).
Ari is seemingly the only one who cleans the house so probably is hygienic and therefore has a skincare routine, while Sander clearly doesn't wear sunblock.
Ari is studying a serious degree so is probably fairly temperate, while Sander dicks around all day on TikTok and appears to drink a lot.
Most importantly, Ari is the least involved in the show and is pretty removed from much of the family dynamic most of the time, while Sander is right in the centre with all the Jeanette Narc stress and Jazz cognitive dissonance that involves.
Taking care of herself is why Ari looks fine. Not taking care of himself is why Sander has aged like this since 2018 (L-R 2018, 2019, 2020 then 2021, 2022 and a few days ago).
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As a reminder, this man is
24.
Edit: more succinctly, if he'd taken care of himself and distanced himself from that toxic family then he could have aged like this:
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which I'm pretty sure is what he still thinks he looks like, but still. Maybe his hairline would have even hung around a bit longer.