I've heard (and read) that some can. I'm reasonably certain that some can't. Sometime around 45-50 years old the level of testosterone in the blood starts to decrease. By the time a man is 90 it is likely his testerone level is so low that he not only can't get enough of an erection to masturbate but he loses interest in sex and doesn't feel the need to.
Yes, they can. They do loose interest , but some do not. I actually heard that an 80 year old man can shoot farther than a 17 year old. - dr. Clay Ellis
I would ask your doctor if it is safe first by getting a checkup and asking if you are in shape to have sex. I would think at 90 I would want a health checkup first. As long as everything is working and in good health state definitely.