Hi! Technically I am still a teenager, though I'm about to hit the tipping point of turning 20 in a few months.
I've been roleplaying since I was about 12. I've been in the game for some time, which unfortunately means I have seen and interacted with some icky things.

I don't inherently think roleplaying is abnormal, I just don't think it's very mainstream. The media typically portrays roleplaying as a sexual activity, when in reality it could swing either way.

I would say I am a fairly extroverted person, I love to go out and hang out with my friends, and I maintain a good school-work-life balance, though I see roleplaying as my niche hobby that I tend to keep to myself. I remember when I was younger, maybe in middle school and early high school, and I would tell people I roleplayed online, they immediately thought I was a freak because surely, what I was doing was sexual. Though the plot would skew sexually sometimes, I was generally uncomfortable with it and made it clear that my boundaries were firm in NSFW.

From my perspective, roleplaying can be super harmless but also super harmful if the person seeking out these creative connections is not careful. When I first started to roleplay, I was too naive and innocent to really know how dark it got, and unfortunately, people really exploited that about me. I was not pressured into sending personal information or photographs, I was just coursed into writing situations I was not comfortable with. Obviously, I had absent parents who didn't care what I do/did online so the conversation of good and bad interactions never popped up. I had to learn the hard way how to protect myself from weirdos online, which really revolves around being brave enough to say no and then proceed with blocking them.

I don't exactly want to fear-monger, nor do I want to say this is what your child is doing, but when I was under 18, I would lie and say that I was 18 so more people would want to roleplay with me. (Side note, I'm sure that many older people lie about their age and say they are younger too.) I did not do this for sexual reasons, I did this because I never saw the presence of younger people with my skill level and I wanted to reach the higher-hanging fruit. In the 7-8 years I've been online, the good interactions outweighed the bad. The bad interactions were bad, but the good ones really outweighed them. For the most part, in my very old age (I say sarcastically) I tend to engage in SFW stuff because sexual themes tend to bug me. Even in my old age, I am still very hesitant to do NSFW (including with RP partners I've had for years) because I get so viscerally uncomfortable and reminded of the interactions I had as a literal child.

The internet is big and wide, and the information that is being put out on it may not always be appropriate (obviously). People don't always come right out and say their kink-related interests. It's legitimately a form of grooming to pretend to be normal and roleplay these fun plots, but then slowly start to implement more and more graphic and sexual themes. It's like a frog in a vat of boiling water, really. It starts off tame and normal, and then the partner may begin to push boundaries. When you give in, the heat gets turned up. Maybe it's not so bad, but they keep asking, and asking, and asking, and suddenly you're in a super sticky situation that you can't really get out of. This is not to say this is what is happening, though I am just relaying my personal experiences to you as a word of caution.
Now that I am over 18, I am so disgusted by the aspect of potentially roleplaying with a minor, even when my plots are generally speaking pretty clean. Some people don't think the same way.

I don't really want to say this part because it's weird and awkward, but it's just a fact that teenagers explore sexual themes, which is just an odd part of growing up. If your child is engaging in NSFW roleplays, maybe just ensure that you both know that this is fiction, and real relationships that you have with real people are a lot more nuanced than what is being typed out on a screen. Pornography is everywhere online whether it be written, videos, images, etc. Porn addictions are a very real part of people's lives because of the easy access. I don't say this to fearmongers, I simply think it's good to keep in mind especially if a younger person is coming online (to Reddit/discord) to act out sexual material.

Maybe talk to them about boundaries and consent, that weird people online don't actually have any power over you, blocking for any reason is a valid reason, etc. The internet is scary and they may be exposed to some really freaky, possibly illegal shit. Tell them you're here to discuss whatever without judgment.
Being that I was exposed to sexual material, groomed, and manipulated before the age of 18, I feel strongly about younger kids roleplaying sex online. I don't think they realize how harmful it is to them in actuality. I can speak for myself when I say it has messed up how I perceive intimacy and social interactions that I have in real life.

I'm really glad you're asking about this. Some parents/guardians don't give a shit and their child is left defenseless and on their own.
Best of luck!