I have two affected cavaliers and haven't done surgery for either -- one a rescue so I don't know her age -- but she has only mild symptoms (scratching). My other SM dog was MRId at 1 so I knew he had CM/SM but he didn't show symptoms until close to 2.5.
Some neurologists seem to almost always recommend surgery (more so in the US than UK for example) and it is true that in general, the condition is progressive. But progression is highly variable. Some neurologists feel dogs can be managed medically. Clare Rusbridge, one of the leading authorities in this area, is one of the latter. But early onset CM/SM is a special case as noted below.
To my mind a decision for surgery for me and my dogs really depends on the level of the symptoms and the seriousness of the MRI. If a dog is strongly symptomatic at under 3 the chances are that this is a more severe case and the earlier the symptoms, the more severe the CM/SM is likely to become and the more rapidly it is likely to progress. Some of the damage caused by progression is not reversible so the longer surgery is postponed the more damage that may be done. There is some evidence that the earlier the surgery is done, the faster and easier the recovery.
The surgery won't actually address the underlying cause (in the strict sense) and hence cure either CM or SM but it can relieve the pain and symptoms and prevent further deterioration. In some cases it is really the only option for any medium to long term chance at life (this especially so with young dogs with CM/SM, as the prognosis is far worse for these). Whereas medical approaches only attempt to relieve the pain (though there is also some indication that CSF inhibitors -- drugs that slow CSF flow and reduce pressure in the dog's head/spine -- may also slow or stop the development of syrinxes).
I am not sure what long term effects of medication your neurologist is concerned about but most medications used for CM/SM have very few side effects and many dogsthat have the surgery continue to need those medications anyway -- gabapentin and some form of CSF inhibitor, like frusemide or cimetidine. There are dogs that have been on these long term already with very few side effects. In some cases though the effect of the drug decreases over time.
I have some comparisons of medications vs surgery on my website:
http://sm.cavaliertalk.com/treating/treatingsm.html
There are papers discussing surgery and medication on the same site in the research section under London 06. On Friday of this week there will be presentations in Rugby in the UK on both the surgeries and long term medical treatment.
What kinds of symptoms are you seeing? If they were strong enough for you to have decided to go for an MRI, then they are probably significant. Given the young age of your cavalier, this would be considered very early onset CM/SM and surgery would probably be the best chance for any longer term quality of life -- but there's unfortunately never any certainties with this condition. That's one thing that makes it frustrating.
About 20% of surgery cases relapse within 6 months and either need a repeat of the surgery or are treated medically for pain or fail to recover. In Clare Rusbridge's presentation in London this time last year, she noted that nearly half of her surgery dogs saw some return of some pain and symptoms over time. You can find her paper as noted above. One thing to keep in mind more generally is how many dogs clearly have this condition and never show any signs -- indicating progression is highly variable! But as you have an early onset dog, I would in your shoes definitley be considering surgery as the best option, if there are not other reasons you don;t want to do it (there are many perfectly valid reasons why someone might not opt for surgery -- it's a very personal choice).