Hi,
Just wonder if, how and when to use foot or feet?
I have had so many mixed answers so now I am more confused than ever.

Start with these two sentences:

1) He's a 10-year old (boy).

2) He's 10 years old.

Notice the Indefinite Article in (1), before the noun 'boy' (which may be omitted since the two people talking know who 'he' is.) In (1) 10-year old is used as an adjective/adjectival phrase, qualifying the noun 'boy'. (note the hyphen).

In (2) 10 years old is a description - there is no Indefinite article, and no noun following it.

That is the pattern:
use a hyphen and the singular when it is an adjective
and
use the plural and no hyphen when a description.

Examples:
It's a 40-foot tree.
This tree is 40 feet tall.

This is a 2-metre cable.
This cable is 2 metres long.