I think it really depends. Ticklishness, I've found out, very often depends on the mood of the lee (myself included, although I'm M/mostly ler). If the lee is relaxed, feeling goofy, or generally in a good mood, the ticklishness increases. And again, if they're nervous, irritated for some reason, too tired, it gets annoying for them. After all, let's take into consideration the fact that tickling and ticklishness are also a psychological thing, not just a physical sensation. Also, the mind and body get used to the sensation after a certain amount of time, and the ticklishness decreases.
Having said that, being 36, I've had my fair share of partners throughout the years. Only two of them were what you'd call super ticklish and react with laughter and squirming to the tickling regardless of external factors. One of them was really a tickler's dream - amazing feet, very ticklish all over, and she didn't hate it at all. A perfect sub in bed, but apart from the sexual stuff, she was a total bitch. Anyway, not the point... All other partners I've had have had mixed reactions to tickling. One of them would laugh like a maniac for 5-6 minutes, then just stop, because she was getting used to it and could control it. I've had girlfriends who were extremely ticklish on the armpits and ribs, but had 0 sensitivity to foot tickling.
Last but not least, I think we could all agree that real-life sex is rarely the same as it is shown in porn movies. Same goes with tickling and tickling videos. Yep, it's cool to watch a beautiful gal laughing non-stop for 45 minutes while foot tickled, but isn't at least part of it acting? I'm sure it is.