Is glutamine appropriate for athletes of all ages?
If by appropriate you mean non-harmful, then yes.
If by appropriate you mean quite productive, then it's not appropriate for very many. It's most appropriate for those that are prone to getting ill more frequently from training so much, as it can reduce rate of that occurring. Other than this I don't think it does much.
I don't understand, it's an amino acid. Why would it be useful to get any more of it than you already get in your diet? I heard it is actualy bad to get too much glutamine in your system.