Two forces make early enrollment the financially strongest move. Premiums are priced on age, so a young animal locks in coverage at the low end of the curve.
More importantly, pre-existing exclusions only apply to what's already in the medical record - insure a healthy young pet and the record is nearly empty, so essentially everything that happens later is coverable. Every month of waiting adds history that could become an exclusion.
Puppies and kittens also have a talent for expensive accidents - swallowed objects and broken bones don't wait for middle age - so the coverage isn't just cheap early, it's useful early.