While the 2/3DS family is reaching the end of it's market lifespan, it's still a good system for all who are interested in the DS & 3DS game libraries. It's also considered a more budget-friendly option compared to the Switch.
The DS & 3DS library does run the gamut of age ratings. On the Mature side (17+), games to consider are...
* Zero Escape series, which includes...
>>> 999: Nine Hours, Nine Persons, Nine Doors (DS)
>>> Virtue's Last Reward (3DS)
>>> Zero Time Crisis (3DS)
* Shin Megami Tensei series
* Castlevania: Lords of Shadow
Games are still moderately mature (T-rated; 13+) or more hardcore players are...
* Dragon Quest series (IV - VI for DS, VII & VIII for 3DS)
* Chrono Trigger DS (DS)
* Phoenix Wright Series...
>>> Phoenix Wright Trilogy (3DS)
>>> Apollo Justice: Ace Attorney (DS)
>>> Phoenix Wright: Dual Destinies (3DS)
>>> Phoenix Wright: Spirit of Justice (3DS)
* Final Fantasy III / IV (DS)
* Professor Layton series (DS / 3DS)
>>> ...& the Curious Village (DS)
>>> ...& the Diabolical Box (DS)
>>> ...& the Unwound Future (DS)
>>> ...& the Last Specter (DS)
>>> ...& the Miracle Mask (3DS)
>>> ...& the Azran Legacy (3DS)
There's some other major titles & even the DS's "Touch Generation" series, so everybody can find something they can enjoy.
The ONLY REASON why it's getting the "Children's Only" vibe is that most have moved onto Switch. Typically parents who want to get a game system for their kids, but don't want to buy for a Switch for whatever reason (generally cost, kids have a tendency to break things or kids aren't responsible enough for a Switch). However, it's still a good system to pick up & enjoy.
Hope this sheds some light on the subject.