Dual usage

[creativearcs]

My question is ...

When I was your age, the only thing I could think of is cricket.

Is this sentence grammatically acceptable under any circumstances?

[Casiopea]

Although, at first, is appears to be the problem, it's not; it's could. Here are two non-past uses of could, and neither fits the pattern you want,

To express conditional possibility or ability:
EX: You could do it if you tried.

In offering suggestions or advice:
You could write and ask for more information.

In short, given 'could think', we expect 'was', not is.

Welcome.