Pursuing a PhD in applied math with a large amount of student debt I'm 36 years old and work as systems administrator and lab instructor for a computer science department at a small liberal arts college. I'm also an adjunct math instructor for a community college. I have 1 BA and 2 masters, all from Ivy League Universities. I started college late, at the age of 25. 

I always wanted to have a PhD in applied math and do research, but after undergrad, I felt burnt out and instead chose to work so I could pay off my student loans, which did not go well for me, because the economy was terrible in 2012. Two masters and several jobs later, I landed my current position, which I enjoy more than any other full-time job I've had, but would still rather be a full-time mathematics researcher/professor. 

So I began applying to PhD programs last December. I was offered two TAships. I'll be attending the one I chose starting this Fall. It's at an R2 university. I'm excited but I'm now experiencing cold feet, because my current student loan balance is about $150K. I'm now beginning to think getting a PhD isn't such a smart idea. In fact, if I don't pursue a PhD, I could pay off $65,000 in about 3 years. 

My plan prior to last December was to work at my current job for about 3 years and then find a full-time professor position at a community college or technical college, or to find a full-time lecturer position at a four-year college. I'm thinking I should forego the PhD and stick to this plan instead. Sure I won't be a researcher mathematician as I would love, but it's the smart and safe option. And maybe I can pursue a PhD once my loans are paid off. I'll probably be in my mid-40's by then, but who cares. I don't have kids and I'm not planning on having kids. My girlfriend of 6 years, who I'll probably marry, is unable to have kids anyway. 

Any thoughts/advice? Thanks.
First, taking on a large amount of debt at this point could be a big problem. However, several candidates for president are currently discussing forgiving student debt, a good thing to do in my view. It will be seriously discussed over the next 15 months, at least. It may be that your debt problem will get erased. Whoopie. 

Second, spending your life doing something you aren't happy with is a terrible existence, but you say you are relatively happy now. There is no guarantee that you will land something better, but having already gone to ivy league places that seems like less of a problem. 

Third, ignore any issues about age. You will (hopefully) reach 45 with or without your degree in any case. Graduating after 50 is an accomplishment, just as is graduating at 30. 

My suggestion, worth about two cents, of course, is to continue on applying and see what happens. For a degree program in applied math you probably don't need to make an unbreakable commitment and can keep flexible as you see what happens, both in the general political/educational scene and in your life personally. 

Of course your future spouse has a say in how she wants to live her life, so don't forget to factor that in as well.

But none of your impediments seem to be actual blocks. Be your best.