A friend of similar age can be the best choice

if stranded on an island

There for you for longer than many family members, especially if they're older

Able to relate

Able to bond

Able to have fights:

from fear, exhaustion, hunger

Yet still able to come together and sort out 

things which hold no value on a large scale

Friendship is a true test

Most are bound to their families, whether be it by some biological bond

or similar blood and dna running through you both

But friendship.

You are not forced to stay in eachothers lives because of convienience or societal rules

Friendships take as much work as a marriage

Some are worth it while others are mearly a great learning experience; a stepping stone to the Next Great Person

even if it is painful at the time

You must take a chance in order to maybe have someone amazing in your life

A truly strong person will do this

even if they've been hurt before.