Hello!

Ok, is there like a level requirement to use a trainer. I encountered a heavy armor trainer that is supposed to be "adept", but at 29 skill level she won't train me... How do these trainers work?

It's how high they can train you. Up to 50 - Journeyman. 75 - Adept. 90 - Master. Some of them I believe you have to do a quest for or get on their good side to get training from that might be one of them. How does she respond if you ask for training? Oh another thing to keep in mind you can only receive five trainings each level so you might have hit the cap for this level. Ask her again once you level up if all else fails.

....LAWLLAWLALAWLALWLAWLLAWLAWLLAWL
is this your first TES game?
if so its ok

now the different levels trainers teach at
(Adept,journymen,master etc)
the word is their rank
master means they got a 100 level in their skill, which means they can train you from 1-100 in that skil
adepts are the lowest ranking skill trainers...you cant be trained by them because you know MORE then them in that skill
you can be trained by a trainer at ANYTIME as long as their level in the skill is higher then yours

She doesn't even bring it up... Although guides and wikis say she should.....

Don't laugh at the guy, especially since you also have incorrect information. There is no up to 100 training anymore, Masters can only train you to 90 you gotta get those last 10 points yourself. And there are three levels now, Journeyman up to 50, Expert to 75, Master to 90

what trainer is this?
she might be a guild trainer(have to join whatever guild she is in and can be trained, dark brotherhood,bard,mage,companions

Heavy Armor
Adept: Hermir Strongheart
Windhelm

So the option isn't in her dialogue? Have you been training with others? As I said there's a 5 training per level cap, so you may have hit that level and thus can't train right now with anyone else until you level up.

Nope, haven't used a trainer thus far.

Slowly. . . Mondays to Thursdays.