If a child is showing signs of autism at an early age, 17 months, does that mean they are more likely to be on the more severe end of the spectrum?

Not necessarily: Evaluation of toddlers with signs of autism includes tests of overall development, like a differential abilities scale & an autism diagnostic observation schedule. Level of cognition, degree of asd, & early response to intensive behavioral intervention, like the early start denver model or aba are indicators of outcome. See http://www.Nj.Gov/health/fhs/eis/autism.Shtml to get started asap.

Not necessarily: I find 17 months towards the later end of showing some suggestive findings. Autism is not all one entity although we tend to lump them together under one name. Most will develop many skill sets that might seem unlikely when first diagnosed. Cautious optimism should be maintained as you work with those that will provide needed therapy and direction down this unpredictable road.