I still can't find a good answer for this: to what extent does age matter when it comes to bodybuilding and athleticism? I've been leading a sedentary lifestyle for almost 26 and I'd like to change, is it too late to gain good mass and a good physical strength?
Ed Corney didn't get into bodybuilding until he was 33 (cf. interview with Ed Corney) and did very well for himself winning the Mr. Universe and being a Mr. Olympia contender. You can see a YouTube video of him at various ages here: Ed Corney Posing.
More to the point, muscle strength (and size) begins to decrease in adult men and women largely as a result of simple disuse. There is nothing about being in your twenties or even thirties which will inhibit your ultimate potential as a bodybuilder.
In fact, unless you actualize 100% of your potential you will probably always be able to get bigger and stronger, even into your 50s and 60s and beyond if you haven't used drugs as an ergonomic aid. The reason for this is that muscular hypertrophy in response to bouts of intense exercise - what we call getting bigger- is an inherited response built into muscles. That response doesn't cease to "work" at any particular age just the way your skin's response to sun (getting darker) never ceases to work.
I took my grandmother into the gym at age 72, had her perform 5 sets total for her entire body, each to failure. I kept careful records of weights and time-working and each time we returned to the gym, she was measurably stronger in weights or reps or both. We kept this up for months until I moved away and there didn't seem to be any slow down in her progress.
How quickly you recover from high intensity exercise may decrease with advanced age as may how much exercise you can tolerate. Those may ultimately be the limiting factors in upwards gains of strength. But certainly in your 20s you have absolutely nothing to worry about.
One caveat is- if you reach a high level of strength through drugs, you won't be able to sustain it after you go off those drugs. So you will reach an artifical high quickly, then go down no matter how you train. In this case only, proper training will lead to a decrease in strength.
It is never too late to do something for your health. 26 years is not that old and it's a good thing you want to do something for your health and strength. Before you start, you should collect enough informations about what is best to do in your situation. Especially if you want to get super good and have really high ambitions: don't rush into working out. Begin slowly and take one step after another. You should do that at any age if you start something new, otherwise you can easyly be disappointed because you might ask too much of your body very soon and might even lose your ambitions. You can shape and strengthen your body at any age - even if you were 62 you could start doing something, the only difference would be that you had to take smaller steps to avoid you asking to much of your body.
At 26, most people are in their prime when it comes to how their bodies respond to workout and building muscles. Even if you haven't lifted ever, or done any kinda workout, it's not late to start, but keep in mind, you can't build a rock solid body overnight. To see any transformation, it takes a lot of dedication, hard work, focus, diet, good rest and above all, time. So, start slow, and keep on building gradually. It may so happen that you won't be happy with the results after a couple of months, but don't give up. Note your progress, and see yourself when it's around 7-8 months and then after a year. You'd definitely see the difference, if you are disciplined with your workout and diet. No, it's not late, I'd say it's a good time to start.