If a plant germinates from old seed stock, does it affect the viability of the plant?

I can't speak directly to melons since I don't do any of them but as a general rule I haven't found that to be the case. The germination rate will fall off substantially with old seeds - depending on how old - but IME if the seed germinates it then grows normally.

I'm sure there are a few exceptions to that rule but off hand I don't know of any.

"Aren't thriving" how?

Dave

Thanks Dave! This was my take also. Of the few seeds that germinated, several died for no apparent reason when they were 2-3" tall & the remaining plants are demonstrating a very slow growth rate for a melon. This is my first try at growing Crenshaw from seed, so I may be missing something........