Say more about your in-laws; their age isn't much of an indicator. The fact that you're asking, might be. Both my iDevices are on 15.1 and I'm older than they are. What kind of "safe" are you concerned about? Odd crashes? New/unfamiliar interfaces or behaviors? Frantic "I broke it!" phone calls to you? You know your ILs best, and what might throw them (if that's your concern). It's hard for strangers to say much more than "avoid it - it behaves weirdly" or "It runs smoothly for me" expand (could be my fingers getting dry with the colder weather); occasional dropped calls (could just as well be a glitch at the carrier or in my callee's phone or service); stupid inconsistencies (which have probably been there forever) like the default button for the countdown timer is Stop, but for the alarm, it's Snooze. Bottom line, v15 is a feature add release. Someone who's familiar with the way their phone works today and wants/needs it to stay that way, don't upgrade. If the improvements will outweigh the inconvenience of learning a few new things and unlearning some old habits, then go for it.