Word between young and old
Middle-aged functions as an adjective.
You could also call the individual by the name of his/her generation (millennial, baby boomer, etc.). There are also technical names for people in their twenties, thirties, and so on. But these names are uncommon.
You could also communicate the age of the individual by describing him/her with the key characteristics of most middle-aged people: gray-speckled, beer-bellied, double-chinned, stubble-ridden, white-sneakered, etc.
These might work depending on the context:
- mature
- seasoned
- ripened
- developed
- grown-up