Can you develop narcolepsy after age 40?

Yes, you can. Yes, you can. Although narcolepsy most often begins early in life (teens and 20's), it can begin at any age including in young children and in midde-aged or older people. It causes daytime sleepiness. However, with daytime sleepiness beginning in the 40s, other causes including insufficient sleep and sleep apnea must be considered.
Yes. Narcolepsy typically has its onset in 2 different age groups, teen and young adults. However, like most illnesses it can occur at other times as well. The typical symptoms include excessive daytime sleepiness, vivid dreams, sleep paralysis, and cataplexy (drop attacks associated with strong emotions). As other conditions can mimic narcolepsy, see you doctor if you have concerns.