you are right about them saying "J'ai vingt ans" instead of

but you tend to get the hang of it by time. For this instance， if that makes sense. I hope this makes sense :) ， when people talk about their age， you are right about them saying "J'ai vingt ans" instead of 'Je suis vingt ans". This is because age is considered as an object， "I have lived for many years"， The difference in usage is a little tough to understand initially， and hence the translation becomes "I have gained many years of life" or，。