Split a CSV file based on second column value

I am using Ubuntu and I want to split my csv file into two csv files based on the value in the second column (age). The first file for patients under 60 (=). For example, if I have the following input:

id,age
1,65
2,63
3,5
4,55
5,78

The desired output is: file_under:

id,age
3,5
4,55

file_over:

id,age
1,65
2,63
5,78

I have tried the following code but it removes the header (column names) how can I avoid this?

awk -F ',' '($2>=60){print}' file.csv > file_over.csv

The input file is about 50k rows (lines).