Reading ring widths

A list gives the width of the annual ring made in each of ten years, taken from a core of a tree. You will find the age of the tree, the year of the widest ring, and the years with a very narrow ring.

The idea. One ring is made each year. A wide ring means a good year. A very narrow ring means a poor year, such as a dry one.

Your tasks.

1. Finish age_in_rings so that it returns the number of rings in a list of rings.
2. Set widest_year to the year of the widest ring.
3. Set poor_years to the years whose ring is $2$ mm wide or less, in order.