Yes, he could still get the lazy eyes although they are born to be. Sometimes bad habit of using your eyes will cause your eyes to be red and blurry at the vision. If you keep on living with the habit of using your eyes too much time every day, you could get lazy eyes because of the damage of the eyes nerves. Thus you'd better protect the eyes very much.
Well, it seems that you brother is quite mature. However, age does not equal to immunity to diseases. As you can see, lazy eyes could be resulted from many factors, such as refractive error, which happens to us no matter how old we are, leading to lazy eyes. And in the mean time, amblyopia is another big factor. In a word, whenever, wherever we are, we should try to protect our eye health.
Actually, there is no any definition showed that adults will not catch lazy eye. It is easy to get lazy eye if your mother are myopia or astigmatism. That means your one of brother's eye has much better focus than the other eye. If this goes on for months or years, you should bring your brother to a doctor and receive proper treatment option.