When I started reading, I already thought about how to answer "no", because I was thinking of other students writing recommendations. If, however, they themselves are already professors, they should have some academic reputation with which they can vouch for you. Thus, I would suggest the following steps:
1) Ask them what they think about it and let them send the letters to you first.
2) Ask other people, e.g. your advisor from back then, contacts in the field, maybe even a contact in a graduate committee you know but don't plan on applying to.
3) Get "enough" recommendations to not depend on your former colleagues to much, so that you have the desired number of letters (some PhD programs post such numbers online) and these are but add-ons. As you have been working, and this working time was long enough for your fellow students to get a position as a professor, you might need to explain why you apply for a PhD just now, why you didn't so before and why you don't just stay in the industry. As this is rather important, I put an extra number for it:
4) Get recommendations from your current industry job to show that you are not (only^^) applying for a PhD because you want out of the company or lost your job, but rather because you want to learn, study, better yourself. One or two superiors at your current work place, stating that they advise on you getting a PhD (as you show the necessary skills, etc.) might be just as convincing as your former advisor. If possible, take supervisors who themselves hold PhDs and make sure that they sign as something like "Mr XXX, senior YYY at ZZZ, PhD in ... from ..." to show that they are able to judge your capability and the usefulness for you to get a PhD.
So long story short, I would if possible get letters from your advisor and from your current job. The former colleagues could be a nice addition to that, but I would not put the main focus on them if possible.