Shall I apply again after all my PhD applications are rejected? I am 29. I've had informative, goal-driven, well written SOP, I'm guessing good recommendations. I have a masters in the related field (Computer Vision) with tier 2 publications. I'm in a research engineer job which can not add publications to my profile. I have been rejected by 13 US universities (all not Stanford, Berkeley). I had contacted the professors. Many did not respond though.


How do I ask for recommendations again if I try next year? 
What can I add to my profile in this scenario? 
Are younger students preferred?
Will there be sufficient opportunity if I complete my PhD after 35?
Age is not a discrimination in your age range, there will be opportunities when you complete your PhD but you should ask yourself what kind of opportunities are you seeking by completing a PhD.

If you stay at the same job your LOR will likely be the same in one year, so you have to change something else. I would suggest you reach out to 1-2 professors whose work is closely related to your job and see if you can be involved in one of their projects, and see if this can also benefit your company. I am afraid if you don't build a more direct connection with academia/research your chances will stay low. Otherwise you have to aim lower, less prestigious schools, but then we go back at WHY you want to do a PhD.