There are no real restrictions on issuing a visa when it comes to age. The only restriction is from the school doing the hiring.
The trend is to hire those from about 22-35 years of age. Those over this age do still find employment in Korea, but it sometimes takes longer. So, if you are within this age range, you will be a prime candidate.
If you are in your forties, fifties, or sixties, you are still more than welcome to apply. We have had success in placing your peers. However, your patience will be required in finding a suitable position.( in public school age limit is 55 )