Need suggestions on doing PhD or not
Don't do it.

It sounds like you don't know why you want to do a PhD. You focus on things that aren't reasons, and where you do mention reasons they are very vague, e.g. in a comment you've said that:


  But I will have also opportunities to intern at national labs during my PhD, would that be helpful for my post-graduate career? And their math program is top 50 in QS ranking, sounds so attractive. And I do love research, the main issue is I'm too old and have no money.


First sentence ends with a question mark indicating you aren't sure it's true. You clearly can't make decisions if you don't know if it's true, so you should find out first before even thinking about the question. Second sentence indicates you want to study at that university simply because it's in the top 50 in the QS ranking, which is fine (I suppose we all have different priorities), but are you sure it's worth it? There's a high cost to studying: you earn less (if you earn anything at all), you spend time, you have to relocate, and so on. The same goes for the third sentence. If you're too old and have no money, why are you considering a PhD?

My read of your question is that you're studying because you have no idea what to do next and / or want to avoid the job market, which is not a good place to be. You'll have to get a job eventually - even if you do a PhD, you'll eventually finish the PhD and have to find something else. Until you've figured out what a PhD allows you to do that you can't do now, you should not do it.