I'm trying to add my son as my dependent but it keep saying the birthday isn't valid (its 23/05/2019). Are they not considered a dependent till they're over a year?

You need to put in the date without spaces or slashes. Ex: 23052019

Also, you may want to double check the year in the top left. Make sure it says 2019. If you accidentally went into 2018 it will tell you the date isn’t valid because it will only take dates in 2018 or before.