Stark07's answer here is perfectly correct when at looks at the reasons from Tony's perspective about why he leaves the armour behind, but I think this is worth digging a little deeper into. Tony Stark is one of the most well developed characters not just in the Marvel Cinematic Universe, but in modern cinema today, and his relationship with his Iron Man suits is as fascinating as it is unhealthy.
If you recall the events of Iron Man, you will remember that Tony Stark first designs his Iron Man suit (and later refines it) in order to escape captivity in Afghanistan, where he was being held by the Ten Rings terrorists. It doesn't take many rewatches of Iron Man and Iron Man 2 to see that not long after his imprisonment, he begins to suffer from PTSD (post-traumatic stress disorder) - simply look at his actions in the films, almost all of which are motivated by a greater need to "take control".
By the time Iron Man 3 rolls around, Tony Stark is in a worse state than ever after flying through the portal in The Avengers/Avengers Assemble. It is here we see his PTSD at it's worst state - he is quite literally a mess, having panic attacks if asked to recall the events of New York and spending increasingly large amounts of time building more Iron Man suits, to the point where it is putting strain on his relationship with Pepper Potts.
The events of Iron Man 3, which leave Tony suitless and force him to rely on himself rather than the suits for the first time since Afghanistan go a long way to easing some of the worries he faces - most importantly, that HE is Iron Man, not the suit. The end result of this is that by the time Avengers: Age of Ultron rolls around, we find ourselves in the company of the most mentally sound version of Tony Stark we have seen since his first appearance in Iron Man - at least until Scarlet Witch interacts with him.
Tony Stark's PTSD and his new found ability to cope with it is the reason that he takes off the suit in Baron von Strucker's base. It's a way of showing the audience that Tony Stark isn't as reliant on the suits as he once was - that he is "better", so to speak. He feels confident that he is relatively safe without the suit, and as such he leaves it behind - but not too far behind. He is Iron Man after all.