Many of host families who are currently looking for a Japanese au pair prefer an individual with few years of professional experiences. Idea is that people with working experiences tend to be mature, and they would perhaps do cooking, cleaning and with more time management.
Maybe, yes, maybe no, I think.
Some of our au pairs are full time university students and they are currently living on their own which means they have to cook and clean up their apartments. Most of them have a part time job along with their life as university students. Students need to have a time management skill to balance their study and work. I think including university students or newly graduates without enough professional experiences for your au pair candidates is a great idea.
Many of university students have experienced study abroad or volunteer abroad that gave them an excellent global sense which you may like. Many of those younger au pairs have good English skills which you may want to consider.
So, why not conducting an interview first and see if you like her/him.