redearslider.com

I have one turtle that is one year old, and my friend has a turtle that is about two months old, are they allowed in same tank?

Are they both RES? It's not recommended that you put the two of them together, your one year old may become very violent towards the younger turtle. Why do you want to put them together? For playtime? RES are very happy being king of their own tanks and don't need playmates. If you are taking care of your friend's turtle for a short time, I'd recommend a large rubbermaid container for the temporary vacation home of the two month old.

RES are naturally territorial and chances are the bigger turtle will push the smaller turtle out of his territory. Chances are the bigger turtle will use violence eventually and hurt the little turtle. Its not recommended to have turtles put together.

Not only is the propensity towards violence an issue, but one of the turtles could be ill, you don't want to risk them getting the other one ill. Whenever you're introducing new turtles they should always be quarantined for 90 days.

RES are aggressive turtles. They certainly don't need or enjoy other tank mates, they're happiest with their own territory.