My son started pretty early, probably around 12 for simple lifts, 14 for intermediate lifts, and 16 for advanced pas work (with professional-level choreography). He was always tall and had a lot of core and upper-body strength from having been in sports, so he was physically ready. In fact, as a caveat, let me just say that my son is exceptionally strong.
There were some clear disadvantages to being able to partner so early, however, especially at a studio where there are few boys. I felt like his individual technical training was neglected for a couple of years, because the attention was on the pas and no one was terribly worried about how the guy looked. This problem partly corrected itself once my son was old enough to do the male variations in the role of prince. I also had to advocate for him in this regard and take him for extra classes at another studio. He still feels like his variation work is a year behind his pas work.
Another disadvantage is the emotional politics of the pas. He was thrown into studio dramas that are easier to deal with when you are a few years older. There were tunics that simply had to be changed to match the color of the girl's tutu, and girls' bodices that were too slick to grip and and had to be traded out with many tears. There were parents who didn't trust my son not to drop their precious daughter and said so. There were questions about bruising and whether it was his fault for gripping too hard or hers for not having enough core strength. There was resentment when the a.d. selected a girl because she was the right size for my son. And there were times when the girl was terribly mismatched with my son but was selected anyway because of her seniority or technical skill. There were nerves and blame back stage when a pas was not perfect. Et cetera. This is all to say that there are many reasons that pas work is more productive with mature students and should not be rushed. At 17, he has the emotional maturity to handle this drama with grace, and he can cultivate trust with a partner. When he was a 14 year old boy partnering a 16 year old ballerina, that was not the case.
The advantages to having started so early are that he now has real partnering skills and experience that he can take with him to guest performances, to s.i.'s, and to master classes. He gets excellent performance opportunities at his two small pre-pro studios. When guest teachers and choreographers come in, they are often very eager to work with him. I was very pleased with how his partnering skills were used in the end-of-summer show at his s.i. In addition, as he moves into more advanced training and perhaps into a career, he already has confidence in his partnering, which is as hard to acquire as the skills. So, I don't really know if I regret that he started so early. I think my biggest regret is that he didn't start intense variations work at the same time that he started pas work.