I doubt you are seeing issues in a young puppy though it is always possible. Probably most dogs are diagnosed at around age 4 as this is a progressive disease but early onset cases have been seen as young as 8-12 weeks. I have lots of videos and information at
www.smcavalier.com and there's a huge range of info as well at
www.cavalierhealth.org.
As the symptoms sheet at smcavalier.com explains, the issue with SM is that early signs can look much like normal behaviour but the behaviour tends to be obsessive or excessive in SM dogs. The videos can help you se the difference.
I am sorry you hadn't heard about the condition earlier -- it actually is one of the things you want a breeder to be testing for before buying a puppy.
But many new buyers won't know that without finding that information somewhere in advance, which can be hard to do. Sadly a lot of breeders (as well as pet shops etc) will not discuss key breed health issues with buyers or volunteer health testing information ( in many cases because they do not bother to health test anyway
). Any responsible breeder should at a minimum talk through MVD (as pretty much every cavalier is going to eventually have this -- half by only age 5) and SM (as it seems likely from breed research samples that at least 50% will also have this by age 6-7, though thankfully many will never show symptoms or need medication/surgery). They should also discuss patellas (knees) and eyes with buyers.
A diagnosis of SM can mean many different things. Some have very mild symptoms or none all their lives; some progress, some slowly and some rapidly and some in between; some have a fairly normal life; some suffer a lot of pain; some are euthenised at a young age because the pain is too intense.
Many of our member have cavaliers with SM -- some have lost their dog to it, some at very young ages; others manage their dogs well; some manage but it is more challenging; some easily manage it with few symptoms. It all varies.
For context: I have five dogs; two have MVD and three have SM. With MVD, 1 is on meds and one isn't; with SM, one is on meds and two aren't but will be going on something fairly moderate.
Two of mine with SM were diagnosed around age 1-2; one is 7. With MVD, one got a murmur at 6, one at 7. For most breeds, murmurs are something seen in very old age, not at only 6 or 7. These problems are a major challenge for the future of the breed.