I found the solution! (All right, years have passed since this was first posted - but let me tell you anyway, in case I'm not the only one still playing Sims 3 come 2018 and onwards :-))
I've had this problem with numerous sims that grew up together, so when I saw in this thread that it may be tied to playing tag as children, it made sense.
Some further reading (also elsewhere) brought me on the track of the SOCIAL CACHE! Yes, that one is the issue!
It's called "socialCache.package" and is usually placed in your documents folder, under "Electronic Arts" and then "Sims 3", and you simply have to exit your game, delete it and then start again - and voila! They can no longer remember that they played tag as kids, and hence they very happily engage in more.... adult activities once they've grown up :-)
(If in doubt: No, deleting a cache file does not harm your game. Worst thing that can happen is that it takes a few more seconds to load the first time you start it up again.)