Personally, while I wouldn't say age has "no affect at all", I dont think age is the determining factor of being ticklish.
Case in point: I've known my best friend for 29 years. He has always told me that his mother is very ticklish. She was 38 when I first met her, and he told me this. Last year there was an incident where they were taking a yoga class together, and the instructor was doing a demonstration on his mother's feet. According to my friend, his mother is still extremely ticklish, even now, at age 67, and she went ballistic with laughter, as the yoga instructor was touching his mother's feet, during a yoga pose demonstration.
This may be only one example, but, it is an example, of how age doesn't necessarially have an effect on ticklishness.
Mitch