That wasn't my impression at all. Not sure what makes you believe Kylo Ren is around 18 y.o. when the actor is in his thirties and looks at best mid twenties? The Knights of Ren led by Kylo Ren killed Luke's Jedi students, after which Luke fled into hiding. My feeling from their scenes is that Solo and Leia have been estranged from each other for many years, not just a couple. Don't forget that it's passed into "legend" that Han Solo once fought in the Rebellion (not the "Resistance" to the point an orphan on Jakku know this. If Han had left 2 years ago, it wouldn't be part of his legend that he has left the Rebellion.... Let's consider also the possibility that Rey herself might have been a survivor of this attack on Luke's students, (as a Skywalker or not). She was abandoned on Jakku as a little girl. Even if Rey is a bit younger than her actress (who's 23), it's probably around 10 years ago and maybe closer to 15.

All in all, it's very likely that Ben Solo was born a few years at most after the battle Endor (and not when Leia was around 40...), and in his late teens or early twenties he turned against Luke and destroyed his attempt to resurrect the Jedi Order. We don't know for now how much Jedi training he got from Luke, and we know Snoke doesn't consider him fully trained in the dark side either... but it did look like Snoke had better uses for Kylo's time than train him anyway - but with the apparition of Rey he has now reconsidered his priorities...

My impression is that aside from being severely wounded and obviously in emotional turmoil during the duels Kylo Ren just isn't a very physicaly fit person nor close to a jock (I wouldn't be surprised he is more the geek or intellectual type), and one who may not train his lightsaber skills that much over the years, not really expecting to meet someone to fight against, aside from Luke. Apparently his lightsaber is pretty primitive - based on an "historical" design that still required 2 exhausts vents at the bottom for the extra heat. My guess is that it's the only design Kylo Ren managed to find, for lack of a "modern" Jedi or Sith model to copy.

A lot of people also missed the clues placed in the movie by the screenwriters to explain why Finn and Rey could more or less handle their own. In Finn's case, he has full military training and it appears the stormtroopers are trained now (and equipped) to fight against lightsabers (why, when there was only Luke and a bunch of apprentices now decimated is another question...). He's not a force user and didn't last that long, but it explains why he wasn't inept. As for Rey, she's no stranger to sparring - on Jakku her weapon was a kind of quarterstaff doubling as a walking staff, and she was skilled with it. Add the Force to which she seems to have more an Anakin-like affinity than a slow emerging Luke-style one, the adrenalin, the survivor instincts she's no doubt developed to survive alone on Jakku to the facts Kylo Ren was in emotional and psychological turmoil and physically badly wounded, and it's not all that surprising the duels went the way they did.