Answer: 1.5 Years. (18 months.)
Step-by-step explanation:
Not sure if this is accurate, but I watched a study from a research lab.
It was called the shopping cart study.
Infants were given the task of pushing a shopping cart whilst standing on a mat which was attached to the shopping cart.
The weight of the infant obviously would prevent the baby from pushing the cart, but those younger than 1.5 years couldn't realize this was what was preventing them from pushing the cart.
Only those 1.5 years older and above were aware that they were the ones stopping the shopping cart from being pushed, so they rolled the mat up and continued.
You can look it up for yourself too (the study!)
Around 15-18 months or 1.5 years of age. Hope this helped