First of all,I'm sorry for my poor English but I try to do my best in order to convey my opinion to you.
There is an underlying thought called confucianism, which was derived from China, in Japan.
We Japanese were unconsciously taught this idea by growing up in our society. And "Tyou you no jyo 長幼の序”is at the core of this thought.
The young should respect thier elders.
Basically, it is safer for you to use desu/masu form.
But I think it's OK with you if you sometimes use casual form.

Thank you for reading my poor English. Can I make myself understood in English?