Oh Grow Up!
Ah yes, to be middle aged in the Middle Ages.
A medieval phrase for growing up was to become "sad and wise"
There is very little evidence that would let us talk about being middle aged in the Middle Ages (sorry, couldn't resist). Once a person was grown up, writers did not distinguish phases of life until old age was reached. This is not because people were considered "old" at forty. I don't know why, but the sources simply don't speak of it.
Then came the rest of life.
The grownups begat and raised their own children, and played associated roles. They were first caretakers of the little ones, then they were educators as the children got older.
As the children became adolescents, the parents in their turn negotiated marriage. Still later, they might act as guarantors of loans or advocates at law, or sponsors of nephews. They raised dowries and made wills. As adults they defended the family's interests and sought to extend it where they could.
They participated in community life, whether at a shire court or serving in the city militia.
And so they moved toward old age.