Should I be allowed to buy the first two seasons (more inside)?

You should be if it dosen't bother you and you're parents are fine with it, but unfortunately it has an age restriction for a reason, (In the UK its an 18) so people know what it contains. Instead of people buying it when it has a low age rating and for example thinking it will be like buffy the vampire slayer, (In the UK BTVS is a 15) then having a bit of a shock when they watch it. Unfortunately its down to age, but its for a good reason. :)

If it didn't bother you I don't see why you shouldn't.
But I can see why your mom doesn't want you to have the DVDs... it's not exactly the most appropiate show for a 15-year-old to own...

I personally feel that if she knew you've watched and read the series, I don't see the harm in buying the seasons on dvd since the damage is already done, so to speak. That doesn't mean you should go against your mom if she said no, because she probably has a good reason for her decision, like she dosen't want you to waste your money....or something.

I looked up the TVMA age and its meant not suitable for children under 17. I think thats a bit steep actually, you can't buy the series, yet your fit to earn your drivers learners (or atleast you could when I was 15) lol.