First let me say, good on you for getting back to gymnastics. It is an incredible sport for your all around fitness, strength, balance and flexibility. Don't owrry too much about the fact that your body is changing, all gymnasts will go through body changes as they grow and have to learn to adjust to their new shapes and sizes. There are a few advantages to your new body too, mainly you will be able to develop more strength and will begin developing your adult strength.
Forget the age of the other kids, it wont matter in you are the only one in a class of 6 year olds. Gymnastics classes are not based on age, they are based on ability. If you are embarrassed at being older simply say to yourself. "All it is, is that these grisl started earlier". Don't think of it as the little kids class, think of it as the class for people of your level. Being older you will probably foind you progress a little faster through the lower levels anyway. Being older gives you a few more advantages. You will find it easier to learn and understand what your coach is trying to say, you will be able to concentrate and focus for longer periods of time, you will be able to understand what your body is trying to do better.
There is no rule that says you need to become elite by a certain age, so age has nothing to do with whether or not you will make elite. I mean you could start gymnastics at the age of 20 and make elite at 30. There are incredible international competitiors in their 30's. The reason why people try to make elite as young as possible is often the lifestyle of the sport. Elite gymnasts train 6 days a week for 6-7 hours a day, this is much easier to do when you are in your teen years than when you are an adult and often have to attend a job, make money, look after yourself, have a family and so on. That doesnt mean you cant do it as an adult, its just much less popular.