comma with absolute construction: a child only four years old, he...

[tamebycomparison]

"A child only four years old, he watched in horror as his goldfish jumped out of the tank."

Is this comma use correct, provided that I have mentioned the boy in a previous sentence?

 

[Cagey]

I would say that it is both correct and necessary.

A child only four years old is in apposition to he, and should be set off by a comma.

Here is a thread that discusses a slightly different example of the same construction:
Commas for phrases in apposition?

 

[natkretep]

Yes, the comma is necessary.

I'm just unsure why you referred to having mentioned the boy in a previous sentence. The usage is correct regardless of whether the boy had a previous mention.