quinquagenarian (kwin-kwuh-juh-NAIR-ee-uhn) - adj., 50 years of age, or between 50 and 59 years of age. n., a person who is 50 or in their 50s.
Not a common word, and when it's used it generally seems to be in the looser sense of being in one's 50s rather than the stricter being exactly 50. This is, as you might expect, pretty much straight from Latin: quinquāgēnārius, meaning containing fifty, from quinquegeni, fifty each, from quīnquāgintā, fifty. There are, as you might expect, a whole series of Latinate decade-related words, including quadragenarian for the 40s and sexagenarian for the 60s.
As a quadragenarian, he found himself at odd moments sitting still, thinking of nothing.