There are many young solo travelers traveling the world as backpackers, starting their journey when they are of legal age.
This does not mean that if you are 40 or 50 years old (or even older) it is not possible for you to travel alone.
When you are young you have more energy to go anywhere, you can eat whatever you want without getting sick, and you have enough stamina to sleep in the most unexpected places.
On the other hand, when you start traveling alone after the age of 40, you no longer have the same stamina and energy as in your younger years, but you have learned to be sensible and you also have more money to spend during your trip.
In conclusion, the best age to travel alone is the age you are right now. Don’t expect it to be too late if you can buy your plane ticket right now.
By the way, traveling alone is something you should do at least once in your life.