I don't think he is that much older ,like choi already has a biological younger brother who is the friend of jaein so choi is older than him ,and taeyoon is younger than him means he is same age as jaein or a bit younger than him like 2 or 3 years ( i know it's confusing,read it few times again u will understand i think)
It does make sense..one fact is that author stated taeyoon is 22 y.o and that omega teacher is 30 y.o. My theory was based on that choi younger brother was also a teacher and I though they are like collega hence the same age. I think they could be like sunbae/hobae relationship and they met in uni.
I dont follow the author in x but did the author drop the age gap between mc and ml. I dont mind the age gap just wondering because i think mc is like 10 years older thn ml. Considering CHOI younger brother (i forget the name) is underclass student of uke. Lets say him and uke is one year apart..and at the later chapter we learned that younger brother is the lover of the omega teacher...and him also the teacher himself. We can take that they are about the same age. The omega teacher is 30 year old. So i think that make uke is about 31/32..and ML is 22..so 10 years apart. Neverthless uke looks so much younger.