I'd imagine that hair care in the 24th century would be quite a bit better than just a hot iron.
After all, they can regrow skin with a little wavy light thing,
Then again, Gene Roddenberry was once asked about Picard's baldness. The interviewer asked if people would have a cure for baldness in the 24th century, Roddenberry said that they wouldn't care about the baldness in the 24th century.
There are people that shave their head that can grow hair even in the 21st century. Picard looks better bald, I think it makes sense as a style choice, and in the 24th century there'd be less societal pressure or shaming in being bald if you choose to be. It's already changing today.
As a bald dude let me just say while I have never taken shit for it, and I am also lucky to have a good shaped skull for the wrecking ball aesthetic, if I had the spare cash I would get transplants in a heartbeat.
It is simply a matter of choice, if you want to shave your head because you think it looks good it’s one thing. But if you have to shave your head because you don’t want to look like mid 40s in your 20s, that’s very different.
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That's great. He looks like he has hair like me, which is quite curly IF you take care of it: wash and condition 1-2 times a week. It gets ratty if you don't and the curls start to fall out and they frizz (individual hairs curl separately).
And you can definitely straighten your hair with heat, which holds till a wash.
The old pic below also checks out: grey and white hairs are a different texture and mine are significiantly less curly than the rest of my hair. So it makes sense old Worf's hair is straighter.
It's really not curly, ever. He does have a distinct waviness to it quite often. However, in humans wavy hair can be significantly straightened by just changing shampoo and/or conditioner.
Except his hair was straight the one time we saw Mr Mot cut it (or at least pretend to) and he suggested Worf use a conditioner, so clearly he straightened it without conditioner.
But maybe it was Mr Mot that straightened it to begin with.