I did femur lengthening four months ago at age 33, and I had to stop at 4.2 cm due to bad bone formations, but I do not think these complications reason of age rather than surgical technique as Dr dong hoon Said, I think 30 is not old. If you do it with a top doctor with a slow ratio of 0.5 a day with a reasonable amount of lengthening ( no more than 6 cm femur ) - and of course you need to discuss with your doctor - then I think you will be in the safe side, but if you want to length 8 cm in two or three months and you wish good result, then you need to rethink about it.
All marking doctors will tell you that you can reach 8 cm safely, and once your leg is cut, they start being realistic about the amount of length and recovery time, and you might end up with 4 cm or worse case with serious complications. MY advice to you do go for the good top-notch doctor, never go for marketing and young doctors no matter how much they offer it even for free.