"오빠아무리27개월 갈/같(?)지만27살이야. 뼈가약해질나이라구ㅋㅋ"
=> 오빠, 아무리 27개월 같지만 27살이야. 뼈가 약해질 나이라고. (proper spelling)
- 오빠: (older) brother (as addressed by a female).
- 아무리 27개월 같지만: no matter how(아무리) it seems like (같지만) 27 months.
- 27살이야: (you are) 27 years old.
- 뻐가 약해질 나이라고: it is an age(나이) when the bones(뼈가) weaken (약해질).
It is saying:
Even though it might seem like only 27 months, you are 27 years old.
You're at an age when the bones get weak, lol.
So it is a cute joke at someone older than you with a funny exaggeration.