They are never too old - My 7 and half yr old passed his grade 1 today. You don't need to do agility.
When I started I hold my hand up and thought gundog people were as you, plus I was very suspect of their methods, however, while yes there are a few that fit the criteria, I have met some really lovely people who couldn't have been nicer, even if they were cynical about my methods and my show cockers, but they still supported me and helped.
I used my own trainer and then just took the tests when we were ready. It has helped with the general obedience of my two in a very positive way, that is a great add on. I was introducted to the whistle and from having two cockers that would disappear over the fields/beach etc and came back in their own time (you know that cocker look - I know you want me mum, but I'll come back when I'm ready) I now have two who will recall to whistle very quickly and about 95% of the time I don't need a second command.
Go for it - but as advised check out your trainer first.