The biggest factor or advantage of starting HRT "Early" is that the effects of testosterone on a developing body is stopped dead in its tracks. At 24 that ship sailed long ago. All things passing after mid-late teens is pot luck or as they say around these parts, YMMV
HRT, for the most part, will not make you pass. Help, yes but that is all. Physical changes do take place, but like any sculpture, the end result was already in the stone. The biggest factor to passing is self-confidence, followed by voice and some sort of fashion sense so you can dress appropiately for your age and situation. THe mental or emotional effects of HRT can help a lot in many areas, including self confidence. Even at very low doses and no feminizing effects.
In my early 20's I didn't feel like I could ever pass. I tried and stop my transition experiments twice. Some 30 years later I had a bit of a meltdown when it dawned on me I actually do pass and most importanly I actually am accepted as a woman. (Which leads to the obvious Now What? question) The only physical thing that happened to me to help me pass was gravity winning and me shrinking an inch from my 6ft even height. The rest was finall gaining some self esteem and self confidence with some credit going towards low dose HRT, which didn't help this much in my 20's.