Deciding at what age to get married is really irrelevant because age has little to do with maturity. An 18 year old girl can be very mature and ready to get married because of her environment such as a large family, experience with babysitting, chesed, volunteer work, etc. On the other hand, a 22 year old who has very little experience and is spoiled by her parents, was never expected to lift a finger, had everything done for her can be very immature and selfish and not ready to take care of a husband.
The same goes for young men. I certainly doubt that an 18 year old bochur is ready for marriage, but a 22 year old who has different life experience might well be more mature than some 24 or 25 year olds that are dating and have no clue what they are doing or what they are looking for. Basically still having their parents running their shiduchim and coming back from their dates with no opinions whatsoever.
I met a woman a few weeks ago who told me about her 27 year old professional son who was looking for a shidduch. I mentioned a few great girls right off the bat. She nixed them immediately. She literally turned her nose up at them and one was my niece. Is there any wonder that her 27 year old “great professional catch” is still single?