I think it really depends. Sometimes skin can become less sensitive with age. However, I do know of one case where age is not a factor in ticklishness.
My best friend has told me that through the years, his sister, who is now 43, has never been very ticklish. Even when she was in her 20s, and younger, she might move her foot away, if someone surprised her with tickling, but they couldnt get a genuine ticklish reaction out of her.
His mother, on the other hand, who is now 65, is one of the most ticklish people I've ever heard of. My friend told me he tickled her a few times when she was, say, in her 40s, and she went ballistic. More recently, he told me they were taking a yoga class together, and as part of a demonstration, the yoga instructor touched my friend's mother's bare foot and her toes, and she went ballistic with ticklish laughter. His mother also said that her sister, my friend's aunt, who is also in her 60s, is supposedly very ticklish too.
While age may have some factor in ticklishness, I think it also depends on family history, and individual skin sensitivity. There is no hard and fast rule that every 18 year old daughter is automatically more ticklish, then say, her 45 year old mother. Many factors play into ticklishness, other than just age.
Mitch