Based on this chapter, Mio's age should be 22. (They dated when they were both 20, Subaru turned into a child one year after, and it has been a year since)
Subaru's age is tricky. He was born 46 years ago. Turned into a child the first time when he turned 23 (23 years ago).
Subaru moved next to Mio's house when they were 10 (It was mentioned by Mio to be 12 years ago, and as they are/were the same age)
This is the timeline:
46 years ago: Subaru was born
23 years ago: Subaru (23) turned into a child the first time
-----11 years gap-----
12 years ago: Subaru (10) moved next door to Mio's house
2 years ago: Subaru (20) and Mio (20) dated each other
1 year ago: Subaru turned into a child on Mio's 21st birthday
It is uncertain at what age Subaru reversed to during his first time, and whether did he turn into a child at different points to different ages during the 11 years gap. However, it was implied that he turned into a child once again 12 years ago (hence the moving).
Current time Mio should be 22 and Subaru should be 11.