I have always seen "teenage" as 13-19, but that number is more open to interpretation with this younger generation.
You are right that there is always hope, but the job gets much harder the longer it's put off. And no matter what the upbringing, there's always strong-willed children/teens who will push the boundaries until they get what's coming to them (as my niece has just discovered, now without internet privileges for the foreseeable future until she shapes up her attitude and behavior!)
And yes, the older we get, the harder it is to change our ways - including the way we see others. Which, again, goes back to the fact that the main issues in parenting need to be addressed as early as possible.
Vicious cycle, anyone? :-)