I always seem to find it easiest to critique people at a similar level to myself. I can see the mistakes I make in others work, and hopefully help both of us in the process.
I also attempt to critique the work of people whom I see as better writers than me whenever possible. Reading this critically is akin to reading a ‘good’ novel critically and if I do see something odd or wrong I think this is the proper forum to express it in and not in a review.
I also try to critique beginning authors, many of whom are teens. And this is where I have the most difficulty.