I'm guessing late 20's. He was probably in his mid 20's when he was Kakashi's sensei. I don't think they would let someone be a sensei if they were too young.
Isn't it something like 22? I might be misinformed but I remember seeing a time line somewhere that established that 16 years after the attack, Naruto's current age; Minato would be 38.
You are wrong to btw cause his exact age was not said in the Magna and anime. Next up about everyone said he was in his 20s. Also I read the thread u posted/made and you seriously need to re do it lol as all 3 sannins are the same age 54