As far as I know 'he found his legs' means 'he got ability and independence'. Here it is explained in more details.
But I was surprised - when I read 'My younger son has not studied to go yet' I didn't understand the meaning.

And when I saw 'found his legs', I immediately understood that the meaning was 'My younger son has not learned to walk yet.'

I haven't heard the expression 'in the last year' often (or maybe just haven't noticed it, because haven't known it). And that's why I haven't used it at all.

Now, that I know it I can start to use it.

As I understand, you use it if you talk about several (repeated) events. For example,
I have had 5 tests in the last week. Or, We have had 7 hurricanes in the last month. Compare to something that lasts for some time:
She has studied English for 2 weeks. The weather has been wonderful for a month (or this month).