Do I hurt my ears by listening to music at...

Do I hurt my ears at listening to music at and around 70 decibels?

I can sometimes turn it up to 75 decibels (according to a decibel meter app on my phone from my listening position).

I usually listen at around 65 decibels. And on my iPhone through my Marshall major 3 I rarely go over the halfway mark.

I estimate my listening time a day to be 3-6 hours excluding YouTube watching.

I’m 18 years old and gave a slight ringing in my ears and I am a little bit scared of my hearing getting worse.
I don't know if the general medical consensus has changed since the 1970's, but back then it was said that 80 db was fine, 90 db was pushing it, and 100 db was a sure ticket to rock star deafness.
If you have tinnitus, 75dB is not the cause. Unless it is 75dB real continuous and for hours. But even then that will usually only be a short term loss. Like when you get out of the car after a long trip, the road noise means you need a few hours to get your fine hearing back.

At 18 you should only be hearing ringing after a 105dB rock concert. Even then it should be gone by the next day. But tinnitus can have many different causes, and even be totally unrelated to loud sound exposure. Drugs like aspirin, and even ibuprofen can cause it, although again it should take a lot over a long time. I gave myself tinnitus one time by taking a boatload of aspirin for a bad burn. Point being, do a little research, maybe even see a doctor. 
The OSHA standard is 85 dB weighted average over an 8 hour work day. Employers are required to have hearing conservation programs if workers are continuously exposed to levels above that. 70 dB for the time period you are talking about should not be an issue. 
In my view it’s quite unlikely you have caused damage, let alone tinnitus, with your relatively modest listening habits.

There are a lot of covarying factors involved with tinnitus; some psychological, and of course, there can be purely physical underlying causes that you would want ruled out (such as acoustic neuroma).

But don’t listen to me, seek out a medical professional if you’re concerned.  
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Tinnitus can be caused by many, many things. Unlikely it is listening @ 70db.
I have a NIOSH SLM app on my iPhone. Instantaneous  dB readout and tells you how long at that sound level is safe. 75 dB is safe for hours. You need an ENT and a audiometry test.
In general, mid 80 db for extended periods or higher for less is regarded as SPLs that typically cause ear damage.