Joseph: Yes, but if you’re not ready to get married at 17 but you expect to need 3 years to try, then it is cruel, a waste of time, and possibly an Issur of Kreiva L’arayos to start dating before that time.
The real answer is that D’racheha Darchei Noam and that we know that HKBH did not expect us to get married if a person and/or his prospective spouse is not ready. Therefore we assume that these are not recommendations and end-all rules. Now, recommendations from major Poskim carry weight, but that doesn’t mean there is Halachic force. One is not Over an Issur for not being married past the age of 24. One is not Mevatel as Asei for not being married at that time. It’s not even a Chiyuv Mid’rabannan (though I am sure there are Poskim who do hold that it is a bona fide Chiyuv D’rabannan). It is a strong Eitzah Tovah. (Outside of Yerushalayim, where there used to be a Cherem on being a Bochur over the age of 20 but there no longer is such a thing.