The Thief build I have planned goes up to 65 instead of 50. Will this be feasible? Thanks in advance.

i don't see why not, just invest your time and perks wisely.

What does it matter if your level is beyond 50?

Yeah it's feasible, you'll get 15 extra perks along with more to your health/magicka/stamina. More skills, the better. I think enemies stop scaling at a certain point, so pushing onto late levels like 65 should only make you more powerful rather than having them constantly scale up.

Well, I'm level 59 right now on my imperial mage, and I've been playing for roughly 300-350 hours (Steam doesn't count hours when you're in offline mode).

I haven't used any skill up glitches of any description, just playing and enjoying the game exploring the content, so it will probably take a little while. Depending on how structured you are when you play.

If you go super organized mode and level each skill purposefully one by one instead of just playing whatever you feel like normally, it will be faster for example.

Judicious use of the guardian stones helps too, I expect.

I read somewhere that leveling up becomes much harder and more time-consuming beyond L50. Is that true?

I have a max level character that I leveled up myself (no ogfinium glitch here). It definitely takes a pretty hefty amount of time. It can be greatly expedited though if you level up enchanting early on because you can enchant stuff in ways that allows you to go out and farm certain skills quickly (with a correctly enchanted shield you can just stand there in the middle of like 5 or 10 people and take hits to farm out your blocking.) I will admit that there were times that I turned on the TV that I have sitting next to my big screen and just kinda kept an eye on the game so I didn't die. But pretty much just stood there. I can understand that this doesn't appeal to most people but it makes the leveling process ALOT faster if you just focus on one area and max it out before moving on to the next skill. Let me know if you have any questions about the specifics of how I farmed out certain skills because there are ways to get some of your skills up EXTREMELY fast.

To put all this in perspective, Ruben Thomas is a 59 after 300-350 hourse of game play. My first character was 48 after about 150 hours. My max level character was level 81 and a half within 200 hours. It's all about how you go about the process of leveling. If you really want to get there, it's definitely doable, but you're going to have to focus on certain areas. I mean, who the hell wants to stand in front of giants on master mode (because your armor leveling is based on damage taken) and just let them hit you (each hit was taking away almost ALL of my health) and then heal yourself. It's kind of a pain and nervewracking loosing that much health with each hit but it levels up your armor as well as restoration and it's things like this that are a pain but they make you level up MUCH faster.