You've just misinterpreted. Typically, someone notices that one's argument appear to be from a very inexperienced viewpoint. People tend to assume that a discussion will occur between someone of similar experience levels than themselves, so they get slightly confused when what they're reading doesn't agree with that assumption. From there, it's easy to check a userpage to see how old someone is, at which point the confusion makes sense.
It's not that a younger person's youth is inherently damaging to their point, it just appears that way from an outside and inexperienced viewpoint. Unfortunately, I can tell from YOUR argument that you're still young enough to relate to those who feel slighted when someone dismisses their opinion as a result of their age.
Obviously, what I've said is not true in all cases. Nothing ever is.