I don't think curiosity diminishes with age, even if the ability to explore that curiosity may. You can still talk to someone and find out if they are interested in experimental technologies, what they choose to tinker with in the limited discretionary time they have, and so on. You can ask about their opinion on new technical developments and whether they follow any trade publications or forums and the coolest things they've found from those places in the last few months.

Since for programmers at least, such information is not hard-separated from the course of their normal work, they will usually have the opportunity, if interested, to be at least partially informed about such things and have some sense of the overarching zeitgeist. For instance, a C# developer would be fully justified in reading about the changes slated for the next iteration of C# during his/her day job, especially as that iteration neared and achieved release.

On top of this, professional engagement and awareness IS something that deserves out-of-work nurture time, even when you have a family or other demanding non-work obligations. You don't have to have an impressive free time project like a hand-built CPU, but expecting someone to put in a few hours a month to tinker, learn, or explore new things related to their field is not unreasonably demanding IMO, even for experienced/busy professionals.

Obviously "participated in a MOOC [Massive Open Online Course, btw, for those curious]" may indicate curiosity (though it may also indicate someone trying to short-circuit their lack of skill by gaining Yet Another Credential to ride off), but it's far from the only signal.

The best employee is an experienced person who has retained their intellectual curiosity, even if they don't have the time to fully exploit it.