will lack of experience make it impossible to change my subject of study? I'm 33 years old and I have my master's in physics from a relatively good university in my country. 

I have 6 years working experience as a foreign commercial executive which I hated every minute of it, but I needed to make some money. I have to add that I have visited different factories in different countries during the missions I went.

About two years ago, I resigned and diligently went for learning everything about electronics and microelectronics which were always my desire. Now, I feel quite confident in embedded systems programming and everyday I study so hard whatever related to digital electronics but I have not yet applied for a job in this field. 

I know this is what I always wanted.
I have average grades in university and good GRE.

What are my chances to get admission for PhD in electronic engineering in one of US universities?
Age won't matter much. 

"Experience" may matter. Any program has prerequisites, and there's really no way to jump to PhD without that essential knowledge.

I don't think you're going to get into the PhD program without first getting a Masters or having equivalent work experience. The Physics background plus independent study may be able get you into a Masters program.

I suspect you'll have better odds if you do the Masters in your own country, and then apply for a PhD program (in the US or elsewhere).