When should you be doing baron? It spawns at 15 minutes but what a good time to start considering baron or preventing the other team to get it. Sometimes I see a team around lvl 12-13 starting baron after killing 1 or 2 people but it doesn't end well if the other team challenges it since you take so much dmg from baron. What is a good time/champion lvl to do baron?
This is a very broad question . You're not looking for a time or a level, but rather a one condition: You know where enough of the enemy team is that you're sure your team won't get jumped while killing Baron.
Typically, such awareness comes in the form of just having killed a few of the opponents. Then, you know exactly where they are: waiting to respawn.
Outside of that, ward placement and other general strategies for gaining map awareness can give you an idea of where everyone is. If you see three opponents on the bot lane countering a minion advancement, you can be pretty comfortable with the idea of going for a Baron kill even when you're not sure where the other two opponents are.
Keep in mind, however, that on top of knowing where they are, you want to be sure they can't get to you in enough time. As your team gets higher in level, they will need less time to down Baron. You'll want lots of time in earlier levels and possibly as short as 10-15 seconds at higher levels.
There are a few things that greatly assist a successful baron. Things like dead enemies and pushed lanes can force the enemy to play defensively, giving your team enough time to kill it uncontested. But you'd want to make sure that the enemy doesn't have sight over baron, and you'd want to make sure you know where all the enemies are at all times if possible. Having someone ward baron with a pink ward can counter enemy wards well, but even having someone scout it with an Oracles a short time before doing baron can remove any annoying wards.
In terms of champion level, it can be done at 11+ really, maybe even a little bit earlier. It depends on how much map control you have, and how many people you have alive and ready to do Baron.
In addition to the great comments regarding warding, knowing enemy team location, and allowing enough time, there are a few tricks to doing baron efficiently.
Baron will attack the closest champion. - Unlike all the other jungle creatures who attack the champion that first damaged them, Baron will attack the champion who is physically closest to it. Keep this in mind when designating a person on your team to tank the damage, or when trying to distribute the damage more evenly. This obviously does not include Baron's spells such as the knock back, knock up, or dropping acid.
One of Baron's spells is dodge-able. - Specifically, the knock up. You will get a warning in the form of some green bubbles forming on the ground under your feet. If you move quickly enough before the bubbles burst up, you will not be knocked up. This is especially critical for tanks that rely on life steal and auto attacks to maintain there health, as any interruption means more damage taken and less life restored.