Prophet for all mankind refers to the idea that the Prophet is a role model for everyone. He was brought down as the seal of the messengers i.e. the final one to deliver God's message. That is where the phrase stems from.
That still has nothing to do with cultural practice. Things are not gonna be exactly the same as they are now when we're talking over 1000 years ago. It would be illogical for any rational being to think that by adopting his teachings and practices, the first thing you'd think about is whether or not it's ok to marry a 9 or 10 year old. That is outdated practice. The wife that he actually married for genuine love in the traditional ambit of a marriage was an older woman.
What isn't outdated is the way the Prophet treated women and children, his patience, his humility, his love and care for animals, his mercy in battle and so on. These are learning lessons that anyone would benefit from in today's society.