They go off gradually until you make a mistake big enough for it to be like a light switch.

Seriously, the decreasing grip is subtle. As the tires decrease in grip you will likewise back off of the grip you are asking of them. You do that subconsciously and also consciously. The subconscious part is just the "feel" you get from the tires. The conscious part is if you have a moment like a slide you weren't expecting. If a tire has a dollars worth of grip when it's new, you're wearing that grip off a penny at time, and not ten or twenty cents at a time. That's why I say as grip decreases its a subtle thing. The best way to feel that is to mount brand new tires in the middle of the day. You should feel a big difference then.
Your question was mostly about the age of the tire though and based on your description I wouldn't be too worried about that. If it feels good when you are riding on it run it. When you lose confidence in it change it.