If you can be bothered to correct them, you don’t need to tiptoe around it, just say, “I think you are wrong” and briefly say why.
Exactly so. ....
Hi there, like you, I am researching ...... and ...... and I notice you mention the 1789 birth details for ...... and their subsequent marriage, just three years later, in 1792.
May I ask you to re check and let me know the outcome, as I am sure it would be impossible for any person that young to actually marry.
Cheerio for now, looking forward to sharing some images from my research with you,

And of course before you send it, make sure that the person you think was 3 years of age .... well ... make sure.... e.g. if it is based on a baptism, are you absolutely positive the baptism is for a baby ... and parents named, no others of similar name or occupation, in that locality, baby before and after in a pattern, mum not too young or too old,.... check, check check.... and ask on RChat for fresh eyes to check too.
JM.