First we have to comprehend the noun "not a junior any more?"
Typically a person has at least one "not a junior anymore." Most of us have many. Adults, especially when they want us to act like old boring people, like to inform us that we are grown up. I suppose you could call any one or all of those events your personal, "not a junior anymore."
A typical situation for a known Laser sailor might happen while a registration process in occuring for a local junior sailing event.
Examples and answers to your question:
1. I could walk into the clubhouse at most any sailing club in Texas, look at the registration table and see a friend of mine. As I approach the table it would be reasonable to expect the person to say, " What are you doing here old man? You are not a junior anymore."
In this case my "not a junior anymore" would hapen while I am in my fifties. It has probably happened at least a few times while I was in my twenties, thirties, forties and fifties.
In this case I would suppose I had to be obviously older than anybody called a kid before I my not a junior anymore.
2. Some kids are big for their age and some even grow facial hair at a young age. Some young women develop and appear to be much older than they really are. Sometimes a fifteen year old who looks twenty can have a not a junior anymore.
3. If your parent has the same name you do and you do something really bad, your parent might attempt to separate from you from the family reputation. Generally a parent will wait to use such a comment until the child is old enough to comprehend the name Junior. Pooping in your diaper probably does not rank as a sufficient provocation to earn a not a junior anymore. So, I imagine the age to be before that particular not a junior anymore is at least five or six.
4. If you are a lawyer and they put your name on the door that would be a not a not a junior anymore. This moment happens to various lawyers at various ages and depends upon a ton of factors. Some lawyers never have the moment happen.
5. The junior senator feom a state is gerally regarded as the one who has been in office less long than the other. You need to be 30 to be elected senator and the other senator would probably be there first so you would need to be at least 32 before you could have a not a junior.