I am 30 years old now and I graduated with a Masters Degree in Mechanical Engineering when I was 26. I have worked in the industry after I graduated, in 2 companies for one year each and then in 1 company for the remaining 2 years. In the last company I worked mainly in Machine Learning. Now, I am motivated to pursue research in this field.
Will my age and the fact that I have a gap in research after my Masters affect my chances of getting into a PhD program ?
At least in the United States, discriminating based off your age would be illegal, so hopefully that would not affect your chances. I started my PhD program at 25 without a masters after working for a year in an industry that was only tangentially related to my program.
I would also not expect the 4 year gap in education to make an impact. I've known multiple people who have done this after working for a number of years. If anything I would expect most programs to see it as a positive. Many people coming into a PhD program directly after an undergraduate degree would have little to no experience in the area, and as such would have a higher risk of deciding that the field is not for them and not completing the program. Somebody leaving a full time job to pursue research in the field is clearly making a deliberate decision to do so because of their interest in the work. See also responses to this very similar question
Will a 5 year gap between Undergrad and PhD hurt my chances?
In my opinion, it depends. I started my PhD around your age and I found some schools/advisors appreciated more mature students b/c they are more professional and focused (b/c they know why they are there as compared to the UG who may have just rolled into grad school b/c they could). However, I know some programs had unwritten rules of if you were more than 4 yrs out of school they preferred not to take you. So much of applying to PhD programs is department and professor dependent. But don't worry, for the overwhelming majority of schools I think is completely a non-issue.
I myself did not go straight from my MA to my PhD (sort of). In my case, I graduated with an MA at 24 and then was admitted into my PhD program at 32. In addition to that, I had already been through another PhD program (which I had started immediately after my MA) and had dropped out because it wasn't a good fit, and in that sense I felt like I had even more working against me.
In the end, I was never under the impression that the gap period, lack of research, or even a prior PhD experience hurt me. I mean, maybe it did with the programs that rejected my application, but I still got into a few good programs. I would also add that when it came to actually going through the PhD program, I strongly believe that my age and life/work experiences made me a far more mature and ambitious graduate student compared to others in my cohort, which is not only something that all of my professors recognized, but it also allowed me to progress through the program very quickly.
With that in mind, I think you can leverage your work experience to your advantage. After all, the pursuit of a higher degree is not just about graduate student life and producing research, and faculty (should) know that. I think they're also looking for applicants who they can envision actually getting through the PhD program in one piece and in a timely manner, with a degree and job offer in hand at the end of it. I think having prior (related) work experience is a good indicator that you have the potential to accomplish all of that if you can connect those experiences to being demonstrably productive, as well as to your research interests. Basically, if you can think of your gap period as an advantage rather than an impediment, it shouldn't be too hard to write a convincing narrative in your application that argues just that.
