It is impossible for us to help you when you don't write which country you are interested in and you don't let us know what nationality you have.
Based on the visa designations I would guess that you are talking about student visas to Korea. For this kind of detailed visa questions I always recommend that you contact the Korean Embasssy in your home country or the Korean migration authorities since they are best equipped to give you a correct answer. Visa rules can differ based on your nationality. We help students from Europe and America etc to study in Korea and for them there is no age limit that I am aware of on these visas. We have had students that are 35 years old on D4 visas.
Read more:
Student visa to Korea
Study in Korea
List of universities in Korea
Language schools in Korea