He should be about 25/26. It's never directly stated how old he is (or anyone else really except for some special cases)
I don't think that to be the case. MXY has at least to be close to age as Jin Guangyao given the fact that he had molested him in the past. And Jin Guangyao is pretty close in age with LXC so MXY and LWJ should be pretty much the same age as well.
Nope. LXC and JGY are not of the same age. JGY is much younger. And MXY is younger than JGY.
But LXC is also older than LWJ.
LXC is only older than LWJ by 2 to 3yrs. JGY is either the same age or younger than LWJ. As previously mentioned, most of the characters age weren't explicitly mentioned, so some hardcore fans deduced the characters' ages using 'clues' mentioned in the novel.
How old is Mo XuanYu?
Lan Wanji and Wei WuXian are the same age (35 for LWJ, 22 for WWX when he passed away)