Holly is Now Karen Wheeler's Age

Season 5 starts in 1987 and Holly is ~10 years old, meaning she was born ~1977.

After the demogorgon attack when the Wheelers are taken to the hospital, Karen is described as a 46 year old female. ​​​​

So at the end of S5 (1989), Karen is 48. Holly would be 48 today; 49 sometime in 2026.