When referring to the time period between the years 500 and 1500, the epoch of the Middle Ages is capitalized, and that expression takes a plural verb.
The Middle Ages were blessed with some extraordinary painters, mainly depicting religious scenes.
When talking about a person between the ages of 40 and 60, we call that person middle-aged, and that word is hyphenated.
Which reminds me of another funny, true story. My son Daniel once referred to me as being middle-aged, and I took exception to that with great huff and puffery. Then he reasoned with me and said: “Mom, if you live to be twice what you are now, you’ll be doing pretty good. That puts you in the middle now, doesn’t it?”