With parrots, there is no way to determine the age of an adult bird. With a few exceptions, most parrots will have their adult feathers and colors anywhere from 12 weeks to a year of age. After that, age is just a guess.
Birds have different activity levels and possibly this bird is just a calm bird. He also may not be fully settled in. Without knowing his history, he possibly has been in more than one home and needs more than a few months to feel like he is safe and secure with you and won’t be uprooted again.
If this a tame bird that you handle? Does he have plenty of toys in his cage, maybe a swing – things to interact with and encourage activity? Is he on a nutritionally balanced diet such as pellets or the diets we make? If he is just eating a loose seed mix, then his nutritional needs are not being met. Many birds take a while to eat veggies, and some never will. So it is important that their daily staple diet is nutritionally balanced. You may want to take him to an avian vet to make sure there isn’t something going on with him that isn’t obvious – birds will instinctively hide signs of illness.
Here are some links that may help you:
Bird Food Guide
Bird Foraging and Play
Thank you for asking Lafeber,