The beards get ripped off and eaten so much, I do not use it to indicate age, other than when I see one as long as that one is, I know he is over 2 by a bit. My 2 year olds have short 2-3 inch beards....But that could be because my hens love nibbling on a beard...
A 2 year old tom can have a very good beard if the hens allow him to grow it out. My first Bourbon Red tom had his best beard as a 2 year old. He had a double beard with the longest being approximately 8" and the second beard was approximately 6".
My Slates have rarely been able to maintain a beard of more than 2" no matter how old they get. Those hens appear to like their males well trimmed. The males stand and take the trimming as if they like the process.