This is not really true; your hairs don’t know how long they are, they grow for a while and then slow down and fall out. (Iirc they do this by growing a thinner section of hair that easily breaks off). Each follicle repeats this cycle over and over.

After a while this will result in an even hair length that depends on the length each hair grows in each cycle.

I knew something was up. My chest hair has always been the same length. I had surgery a few months ago and had it nearly all shaved. It grew back so quickly and I thought to myself "if my hair grew back to normal length in a month, why isn't it 10" long in a year?". It seems certain hair must have a "terminal length" like you suggested.

My understanding on this was that it's more about durability so your terminal hair length is when on average your hair will fall., not their maximum growth because the hair follicle know when to stop.. as far as i'm aware it's not an intelligent feature, just look like it

>and to re-grow when cut back.

it doesn't actually do that, you are just experiencing hair life-cycle.

hair grows out to x length, and then stays that long for however long (head hair is ~2 years, body hair can be as short as two weeks), then the follicle "dies", the hair falls off the follicle, and then the follicle regrows itself and does it all again. If you cut a hair in half it will be half as long for the rest of the life cycle - this is why regrowing cut head hair takes 2+ years to get to full length (although head hair is a bit special because it doesn't have a cap length), but e.g. chest or leg hair can recover in a month or two, and will APPEAR to recover even faster, because half your hair was at the end of the its cycle when it got shaved.

Depending on the place, its about style and beard quality also

I sort of worked this out from watching my dogs hair grow back. Either it falls out at a certain length (and regrows) or just stops growing.

I can't even remember the last time I had a haircut. My hair just stops growing at a certain length.

I don't really have to shave, either. My facial hair also has terminal length. I just trim it to keep it neat.

That fact would only make beard competitions more silly. It is not an action (or lack thereof) that has you succeed, but genetics.

>learned from a barber that hair has a ‘terminal length'

Dont know how much of this is accurate but when l misspelled harebrained (foolish) to "hair brained" I found this interesting talk:

Your hair when it grows at full length gets phosphorus, calcium and vitamin D, making memory more efficient and gives you physical strength and better stamina.