Other forms: quadragenarians
A quadragenarian is a person in their forties. If your dad's brother is 42 years old, you can introduce him to your friend as your quadragenarian uncle.
Quadragenarian is just a fancy way to say "40-something." Anyone between the ages of 40 and 49 is a quadragenarian, and you can also use the word as an adjective: "My quadragenarian math teacher is not as cool as my 72-year-old art teacher." Quadragenarian is from the Latin quadrāgēnārius, "containing 40," which derives from quadrāgintā, "four tens."