Hey Pelickanpete
I know plenty of FOs who where only taking on in there 30s but if they dont do it i would assume it is because they are looking for potential future Captains.Not that age would be a factor but the older you get the more difficult it is,
I can only assume this is their reasoning.
All the best
I sincerely doubt the age being over 30 has anything to do with whether you are a captain or not, FR min requirements is only 2000hrs iirc and that can be tapped out in just over 2 years which is a good 30years of service available.