At some point, you're no longer growing up, you're aging. But no one can pinpoint that moment exactly.
After graduating from college, finding our dream careers, children, marriage, and completing (most) of our milestones, growing can feel like aging. Now, you have discovered your true self, your hobbies, interest, and have probably traveled across the globe, and suddenly you start questioning yourself about what else is there to do. At this point, you are no longer growing up, (physically, we stop growing at age 25).
According to this quote by Richard Linklater, there is no exact age to feel like we are aging, it can happen to a person in their 20's 30's or 40's.
Whenever we feel like we are 'aging,' it means that we do not have enough goals. The goals we achieved throughout our lives are 'personal". After making mistakes, and accumulating wisdom and knowledge from this world, it is time to share them.
It is important to keep busy at any age, set new goals and work towards them. Start a blog to share your knowledge, volunteer, write a book, help others, or even go back to and study a subject of your choice.