My headcanon is that due to the futurism the Pokemon world displays (holograms, pokeball technology, conversion of mass into data via PCs, etc), their genetic health and screening technologies have surpassed our own world too in equal measure. Therefore, all the Pokehags should be able to remain fertile for far longer than they would in our world, and you'd be able to avoid genetic defects in embryos caused by older age thanks to advanced genetic modification and screening techniques.
tl;dr, Drasna will be sexy even in her 90s and I will still be making babies with her.