There are no best age to get graduate. If you want to study then you can do graduation at any age. You have to keep in mind to join a graduate course you need to be at least 17 years of age to be eligible for admission. Apart from this its always optimal to get graduated with in 25-30 years age depending on the course you are studying. The reason behind this is because, from the employability aspects, most companies want employees to be at a tender age so that they can give the maximum output. But that doesn't mean at a greater age, it doesn't matter. So your question is quite sensitive and it can be explained in different aspects and above I have mentioned some of them.

I hope this answer helps. All the very best for your future endeavors!