Not at all. Some people do not start boxing until their 30s or 40s.
If you're looking to use Boxing as a form of exercise, then you can start Boxing at any late age.
If you're looking to make a career out of Boxing then starting at the age of 18 is normally too late, because most professional Boxers started Boxing at a early age and start their amateur careers in their early teens.
There is a few exceptions however, Hasim Rahman, the former World Heavyweight Champion started Boxing at age 20, had only 10 amateur fights before turning professional.