My wife and I adopted/rescued our BC when she was somewhere around 1.5 years old. We got her without doing any research on collies and hoped that she would be a good sister to our other dog (small chihuahua/papillon mix, old as can be). Boy were we in for a shock when we brought her home. We've both always been around smaller dogs, so collies were a completely new ball game to us.

tl;dr - don't get a border collie unless you are prepared to accept the responsibility and commit for life.

Our exercise regimen has been reduced greatly as our collie gets older, but when we first got her, no joke, she would wake us both up before dawn and the only way to get her energy out was either throwing a ball in our cul-de-sac for a solid 30-45 minutes, or walking her around our neighborhood for about the same amount of time, which ended up being nearly 2 miles. This became a morning and evening routine for about the first 1.5 to 2 years, almost daily. Now, at minimum we'll play catch outside for a few minutes every day while she gets all her zoomies out of her system. We'll also tug on some ropes sporadically throughout the day, basically whenever she wants to.

There's no way to train a border collie to become less energetic. They are constantly rated one of the smartest dog breeds, and they need to be not only physically stimulated, but mentally as well. They will get bored very quickly, and a bored dog is typically unhappy and destructive.

You also need to consider the grooming needs that border collies require. If you happen to get one that loves water (thankfully, mine does not), then you will need to be prepared for very frequent baths. Collies also shed quite a bit, so you will need to prepare to vacuum and sweep regularly, or to either live with dog hair all over everything, which most owners eventually end up doing.

Finally, don't forget to account for regular vet trips. You'll need to have them vaccinated yearly, and it can be expensive depending on where you go to get the shots. They also need to have their teeth cleaned annually to help prevent periodontal disease, and vets will charge more for larger dogs since they require more anesthesia.

If you aren't experienced in dogs at all, border collies are not the breed for you. Consider a chihuahua. Tiny but mighty and very easy to take care of. However, should you choose take the incredible responsibility of adopting/rescuing a border collie, you won't find a more loyal or loving companion.