Based on all the stuff I've read, you would have to conclude that mercury stays in the body, accumulates, and "preys" upon genetic weaknesses. Because young persons are classically stronger "medically", they will be less inclined to show symptoms at that age unless there is a significant weakness or they are exposed to large amounts of mercury. This is also why behavioral issues are the first to show in most of these cases.

The mercury in a thermometer is elemental and not methylated. This means it will not pass directly through cell walls (basically) and so it is considered much less toxic. Nevertheless, some of the elemental mercury that does enter your body will become methylated at some point and hence become a very destructive force.

Do I believe that mercury you played with as a child could hurt you later in life? Absolutely.

Actually, it hurts you continuously for years and years, and as your body breaks down with the continued assault, and with the likely exposure to additional mercury and other toxins, the symptoms begin to show.

For example, in my conversation with lab managers, they found that most people with amalgam fillings placed in teen years start showing symptoms in their mid thirties (on average). Of course, this is a little different than the context here because this is a higher-than-normal continuous exposure, not a one-time exposure.

As far as a list of symptoms goes...

It would be the the same or very similar than it is for other types of mercury exposure, like amalgams or vaccines. Here is a list or two that can help:

What Mercury Does To You

Diseases Mercury Can Cause

It's not a clear but situation, but I hope this helps.