Between age of 16 and maybe up to 22 I took DXM possibly 100 times.
During periods of most frequent use I would say my memory, motivation, possibly other mental functions could have been slightly impaired. No noticeable long-term effects I can attribute to DXM. On the contrary I experienced plenty of negative consequences using MDMA/MDA, cocaine, amphetamines, and from tripping way too frequently.
Now I am nearly 29 and haven't tripped in a year and the only drugs I use now are methoxetamine (low antidepressant doses) and Piracetam (on occasion). Oh yeah, weed and alcohol
Issues I have that I suspect wouldn't be as bad if I had not binged or overused a variety of substances: low motivation, some depression, poor finances, trouble concentrating, and a shitty memory.
But yeah, I wouldn't blame DXM at all for this. Much more likely and seemingly attributable to abusing speed/coke (especially a lot of speed), mdma/mda, and tripping too frequently.
NDMA antagonists like DXM don't seem to cause much long-term trouble. I abused the shit out of Ketamine for nearly 2 years and all I got from that was temporary schizoaffective disorder. Which went away after I stopped using it for a few weeks.
So yeah your cousin is fine but it's not a good habit for him to start taking heavy drugs at such a young age. Don't want to go down my path.