Is it wrong to use "would wear glasses" in the meaning of a habit in the past?

Question

I was learning about difference between "would" and "used to" to talk about past habits and tendencies. My grammar book says that both "used to" and "would" can be used in this sense and often are interchangeable, but an important difference between them is that:

You can only use "used to", not "would" for past states.

As far as I understand, it means that I can't use "would" in this sense with stative verbs. Right? Then I took an exercise

I ______ wear glasses when I was a teenager.

a. would

b. used to

The correct answer is "used to" and we can't use "would" here. Is it because "wear" is a stative verb? Is it actually stative? On this and a few other forums I figured out that "wear" should be a stative verb, or at least most speakers claim so.

Nevertherless, "wear" definitely can be used in present perfect continues, for example:

How long have you been wearing glasses?

Whereas stative verbs can't be used in continuous tenses (unless changing their meaning).

And, finally, my questions:

1. Is it true that in the first example we can't use "would" because "wear" is a stative verb?
2. If so, why is it correct to use "wear" in continuous tenses?

Answer 1

I would say your grammar book is wrong in saying possibility a (would wear glasses) is wrong and only possibility b (used to wear glasses) is correct. This gives a totally wrong view. Both are correct, but "used to wear glasses" is much clearer. The reader understands at once that you talk about a past habit. If you use "would" in a single sentence for past habits the reader may get on a wrong track as "would" has a lot of uses.

Mostly "would" for past habits is used in passages where the reader immediately sees that an author talks about past habits as in

After dinner my grandfather would always go out into the garden, sit on the bench under the apple tree and smoke a pipe.

Very often "would" for past habits is used in combination with "always".

See Oald, would no. 12: http://www.oxfordlearnersdictionaries.com/definition/english/would?q=would

Answer 2

Finally I did find an example of usage like that:

He would always wear exactly the same brown pullover and grey trousers.

And in terms of "repeated actions" it does make sense, because when you wear glasses or something else you put it on and take it off repeatedly, so you are not actually in permanent state of wearing it.

Going back to my original example, as far as I understand

I would always wear glasses when I was a teenager.

is grammatically correct, isn't it?

Thanks to @rogermue answer I realized that probably both options are correct, but I would notice that "when I was a teenager" gives quite a clear notion that the sentence talks about past habit, so I don't think "would" is so ambiguous in this case. Though, I'm still not sure if the sentence is ok without "always":

I would wear glasses when I was a teenager.

Is it?

I'm quite sure, that both following sentences are correct, but they have some difference in attitude:

I used to wear glasses when I was a teenager. (Just the fact)

I would always wear glasses when I was a teenager. (It is a lot more emotional and emphasize the typical, repeated action)

Am I right?

Answer 3

Perhaps

a) "I used to wear glasses when I was a teenager" means more "I had problems with my eyes, so I wore them", while

b) "I would wear glasses when I was a teenager" means more "I used to put them on", but now I don't" (despite the fact I (have to) use/wear them at prsent as well, I don't do that)?