Question: Which is correct? And is there a difference?

> 
> 1. He became blind when he was at the age of 50 years.
> 2. He became blind at the age 50.
> 3. when he was at the age of 50 years, he became blind.
> 
> 
>

Answer: 
They are all technically correct, and your meaning would be easily understood by a native speaker, but each sounds a little odd, here's a breakdown:

> 
> He became blind when he was at the age of 50 years.
> 
> 
> 

This isn't ungrammatical, but sounds a little awkward. It sounds fine without "years" at the end. We understand that years is implied since you're talking about age already.

> 
> He became blind at the age 50.
> 
> 
> 

This is another example of having more words than is necessary. It would sound more natural as "He became blind at age 50". The phrase "At age X, something happened" is a very common construction.

> 
> when he was at the age of 50 years, he became blind.
> 
> 
> 

Again, with this I'd just take out "years", and then it sounds fine.

I can't give a deep answer as to why that is, but these are just stock phrases/idioms that take on a life of their own.

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Like the other answer mentions, if I wanted to make them sound even more natural, I would probably say something like

> 
> He went blind when he was 50 years old
> 
> 
> 

or

> 
> He went blind when he was 50
> 
> 
>
There is no difference in meaning. The one I prefer is #2 because it says the same as the others in less words. However, the word "the" was unnecessary and should have been left out. A native speaker would not have included it.

2a) He became blind at age 50.

Idiomatically, the most common way people refer to age is "50 years old". So most often it would be expressed as:

2b) He became blind when he was 50 years old.

That said, I still prefer #2a due to its brevity.