Nope, 19 years apart is a perfectly believable age.
How is nineteen years not believable? My mother and brother have less of an age gap than that.
19 is very believable and a fine age for many to have a baby although not everyone is mature enough at that age.
I think that they mean that it’s not ideal to have a kid at age 19, which really, it isn’t. Unless you get married really young and want to start popping out kids right away. Go for it!