Guppies are tropical fish (found in the wild in tropical or warm environments) They need to be kept in a warm environment to thrive.
To put it simply, if their environment is too cold, they have no biological way of "warming themselves", and will eventually die.

The reason why things do slow down for some fish at lower temperatures, is because more energy is spent trying to keep warm, that could be used for growth or reproduction.

As far as Woolworth's goes, did you mean they are everywhere in the history books, or something else?

Lastly, I seem to remember someone posting in to this site a while back who refered to themselves as a guppy noobie.  Was that you?

Have you considered getting a heater for your guppies?  Maybe you could move them into a warmer location in your house.  I think you'd all be happier.

As a kid, I listened wide eyed as the pet lady at the old local Woolworths (look that store up in a history book) told me that fancy guppies were a Betta x wild guppy cross too!

Some years back, saw some "purple gouramis". Assuming that they were dyed and probably young paradise fish (which I wanted anyway), they were purchased.

In time they grew up, the dye faded and the paradise fish spawned indoors. It turns out they were luckier than those glass fish which are painted. (Many of those don't even live through the painting process.)

Memo to self: Don't buy fish which have been artificially colored. If people didn't buy them, "they" wouldn't produce them.

Sometimes pet shop proprietors are mis-informed too. Those just didn't know any better when given wrong info by a supplier. Once in a great while, as with all businesses, you will also run across someone who just doesn't have the truth in him, but give your person the benefit of the doubt.

G.G.'s advice about verrrrry gradually raising their temperature a little at a time is wise.

Your situation is the flip side of what show competitors do when they "cook" their breeders and fry by keeping then at 80 or 82 degrees and feeding them three, four and more times a day. Water is often changed daily, sometimes automatically. Those fish mature in a few months. They also may be really showing signs of old age before the year is up.

For most of us and our guppies (or other fish) looking up their comfortable temperature range and keeping them somewhere in the middle is best. That might mean you would be keeping them in the lower to middle 70s F or 22-23 C.

If you keep them a little cooler, they will eat less and probably live longer, but they will grow more slowly too. I had never thought about the birthing complications GG mentioned. I wonder if the number of fry dropped, even in healthy birthings, would be less. I would think that they are more vulnerable to diseases if cold too.

An interesting thing happens about this time of year. Fishes which have been kept outside (people get too busy...) are sometimes not taken in until their water has gotten cooler than they would normally tolerate indoors. (That may have been the background of your fish. Ask your dealer if that was the case.)

For reasons I don't understand, tropical fish outside survive and even seem to prosper cool like that. When brought indoors and warmed up, even if very gradually, they seem to age faster than they would otherwise. I can't explain that either, but I'm slowly and carefully adjusting some fish brought in a couple of weeks ago. :)  They are just now being brought to regular room temperature. (They were on the floor in a bucket, then on a stool a foot off the floor....)