You don’t want to overheat in a thick wetsuit in tropical waters, and you also don’t want to freeze or risk hypothermia in a thin wetsuit in chilly temperatures.
This is highly individual. When I’m wearing a full 5mm suit, my buddy only goes for a 3mm shorty.
Also, I will use my dry suit for dives up to 20°C+, simply because of the convenience of being dry after the dive.
It’s the air temperature that usually determines whether or not I wear my drysuit. I can wear thin thermal underwear and dive out in 85-degree water just fine, and thicker stuff in colder water.
What I can’t do is put the damn thing on in 110 degree summer heat. By the the I’m in the suit, I’m sweating so much there’s nothing dry about it.
Came to say the same, my last dive was 64f/18ishC in a 5mm wetsuit with hood and reef gloves and I was toasty and warm the whole time. My 9 year old snorkeled about for about a half hour in a 3mm suit in the same water without getting cold.
It’s not always down to body composition either, which I’ve seen before. I definitely wear my love of cake on the outside, while my daughter is a fit and healthy weight for her age and height.
Meanwhile I would have been cold in a 7mm in those same conditions. When I was in Cozumel with my local club, most of the people were comfortable in either rashguards or shorties, but I started getting cold in my full-body 3mm by the end of each day.