People seem to attribute Kylo Ren's emotional issues to inexperience and youth and assume that in the future films he'll be more composed and in control. Actually, I think it's quite possible the opposite will happen and he'll just get more and more unhinged. Why? Because the reason he's emotional isn't because he's immature. He's emotional because his mental health is an absolute mess.
And why is he so messed up? Well, remember the talk Leia had with Han on how Snoke manipulated Kylo Ren into joining the dark side. I think most people took it as Snoke taking advantage of Kylo Ren's emotional issues, that perhaps he wasn't talented enough and he buckled under the pressure of expectations, which led to a thirst for power. But I think it's actually the other way around. Kylo's emotional turmoil is actually the product of Snoke's manipulation.
In the novelization of TFA, Leia tells Han that Snoke was manipulating him for a very long time, far longer than even she realized.
From the novel:
He had trouble believing what he was hearing. “So Snoke was watching our son.”This seems to suggest Snoke was present in Kylo's life from a very young age. It's very possible that he was a constant presence in young Ben's life, warping the mind of a kid who otherwise would have grown up normally. He was sent to Luke once Leia realized this, but by that time Snoke already had his grip on the kid.
“Always,” she told him. “From the shadows, in the beginning, even before I realized what was happening, he was manipulating everything, pulling our son toward the dark side.”
Essentially he's less the kid who fell into the wrong crowd but rather more like a schizophrenic patient who has a voice in their head coaxing the individual to distrust people, except the voice was a real being manipulating a child. Kylo Ren is a disturbed individual who's been brainwashed since childhood, who's frantically trying to believe that what he's been taught and have done is correct despite the constant nagging feeling that something is wrong. Just like Finn, who still held on to his sense of conscience despite a lifetime of indoctrination, Kylo Ren has an inherent "light side" that he can't get rid of no matter how much he tries. This creates feelings of guilt and inadequacy deep inside that constantly eat away at him and drive him crazy. Years of this kind of emotional turmoil has taken a toll on the guy, making him more and more unstable.
People want him to grow in to the next charismatic, cucumber-cool baddie, but I think it's quite possible that he'll just get more unstable and crazy with time. Honestly despite all the emo teen jokes about the guy, if he was an actual high school teen, he's much more likely to be the kid who stabs someone with a pencil than the one writing poetry while wearing mascara.The root of Kylo Ren's issues is mental trauma. Not because he hasn't grown up yet.
TL:DR: Kylo Ren is a victim of life-long manipulation, which has led to an unstable individual with mental issues.