"craving"? I have never known any man to "crave" marriage. Most men could have married just about any woman they have been in a long-term relationship with. Marriage is an "option" and not a "requirement" for the majority of men. There is no (biological clock) ticking away or anything like that.
Having said that I would say most men start (considering) marriage in their early 30s. This is especially true if they have been focusing on completing college and establishing a career path. After school most guys want to party and enjoy the freedom that comes with being an adult. They're not in a hurry to become their parents!
My advice to any woman in her 20s with her heart set on getting married....She should consider dating men who are in their early 30s who have never been married before.