When you are old enough to love who you marry for who they are and love yourself for who you choose to be. You must possess the skills necessary to determine both.
Perhaps about age 90-110. By then, some people may have begun to mature a bit, if they ever will.
It's any d@mn time they feel is the right one. Cain't nobody decide that.
IMO - between 26 and 30...... you have now enjoyed life enough that you may be ready to settle down with out regret.
There is no appropriate age. You should only get married when you are truly ready for a life long commitment and understand what that means. If it takes til you are 40 then so be it. If you are 21 that is fine too! It should never be taken lightly when committing to be with someone for the rest of your life. You have to understand that it will take hard work no matter how in love you are and you have to be willing to work things out. Nobody wants to end up divorced, that is never the plan when getting married, but if it happens you should be able to say you did your best.