It depends. If someone discloses to me on their resume that it took 8 years to get a bachelors, meaning they put 2000-2008 next to that line item, it's fair for me to ask about it. Besides, professional or not, you want them to ask it if they're inclined to ask it. Not because it's information for them, but because it's information for you. A candidate interviews the company as much as the company interviews them, and from my perspective as a candidate I want to know if these things matter to them and why. For instance, there are qualities I appreciate about non-traditional students - people who started college late, worked full time through, etc. I'm not trying to figure out if you're 45 or not. I'm looking for positive qualities, not trying to shake out the negative. I can distill the negatives when we talk about experience, and those have nothing to do with age or anything else.
Beyond that, when I'm interviewing someone, I know I can't or shouldn't ask certain things, and I don't want to make people uncomfortable or break any laws. So I will often casually mention that I'm married or have a kid if the opportunity arises while we make small talk. I never tie it to a follow up question either, because that's too close to the line for me. But I'm not trying to figure out your situation there either. I'm just putting it out there that I have a life outside of here, that I'm probably going to sneak out (very) early every so often, work from home sometimes to get some other non-work stuff done too, etc. and that I won't hold it against you for being a normal person either.
Beyond that, when I'm interviewing someone, I know I can't or shouldn't ask certain things, and I don't want to make people uncomfortable or break any laws. So I will often casually mention that I'm married or have a kid if the opportunity arises while we make small talk. I never tie it to a follow up question either, because that's too close to the line for me. But I'm not trying to figure out your situation there either. I'm just putting it out there that I have a life outside of here, that I'm probably going to sneak out (very) early every so often, work from home sometimes to get some other non-work stuff done too, etc. and that I won't hold it against you for being a normal person either.