At what age do guys typically start thinking about a relationship that could turn into marriage?
Many people have a hazy future plan that involves marriage/children, but people vary on their age when they make the decision to "settle down". Overall, it depends less on what age a person is and more on the relationship they are currently in. If you've been in a relationship for a couple of years, likely at some point the subject has come up. Unless someone has an overwhelming desire for marriage/parenthood and views it as goal to complete by a certain point in their life, most people will not make a decision about marriage until they're involved with someone who they feel meets their requirements in a marital partner.
Some people have no interest in marriage or children at all. If a person you're dating says they don't plan of getting married or being a father, don't believe you can change their mind or that they may change their own mind in the future.