I'm 25 years old. I've known how the pieces move for years, but only recently I began to learn about piece value, opening principles, tactics, etc. I've read that many GMs started playing as kids or teenagers. This makes me think, "Is it too late for me? I don't plan on becoming a champion/master, but will I ever get at least reasonably good at this game?"

I think so. It's all about how much you put into it. I'm 24 and I feel like I am always making progress and like you, I've known basics for a long time but haven't truly dedicated myself to getting better. I always take breaks. Just use your time to study and learn and when you play try to apply what you've learned. There is still hope for us OLD GUYS!(in the chess world.)

You've answered your own question. You have already decided you aren't going to obtain a GM level of play, so now the question is do you like playing chess for the fun of it? If so then your age is irrelevant. If not, then it's unlikely you've got the time and/or ability to achieve a GM understanding of the game. So play to enjoy and don't worry about the rating. I think far too many people get wrapped-up into the image of chess. For 99% of people, they will never become GM level players and being the majority of chess players there is no need to chase status or ego. Play with friends, family and fellow chess lovers and make this exquisite game a life-long passion. Anything beyond that is ego stroking and isn't required in order to enjoy the game. 

When you're dead or don't have full control of your mental faculties. So unless you get in an accident or develop early alzheimer's, you've got a while yet. 

@J_Young - Isn't part of the fun of chess finding a better understanding of it? How the pieces need to work in unison? How to control key squares? How to develope a counter attack in the center when your opponent tries attacking your king or queen side? The fun part about chess isn't the picking up the pieces and moving them around a bunch of squares. It's the understanding of WHY you're making these moves. If so many people know they're not going to become GM's then why do they continue to read chess books? Why do they pay $99 a year to play on this site when there are many more that are free to use where you can just play the game? They do it because understanding the WHY in chess is what makes you better and it makes the game more fun. Therefore, getting better and having fun playing the game go hand in hand. 

You're never too young, or old for chess! Keep trying and you will get better at it. Don't let age stop you from playing the best board game of all time.

If the "fun" of chess is in the pretending you

are going to be a GM someday, I say go for it.

As the late and great Tupac Shakur once said: 

“During your life, never stop dreaming. No one can take away your dreams.”

But they can take away your life which might negate the dreams part.

if you study and practice i dont see how you can get any worse.

You're only 25, relax, you have plenty of time to pursue your dreams

Clara Peller became a celebrity in her early 80's. 

Grandma Moses started painting at age 70, and continued to do so for another 30 years.

Sydney Greenstreet started acting in movies at age 61 (and what a start, as Kasper Gutman in The Maltese Falcon)

https://en.wikipedia.org/wiki/Grandma_Moses#Painting

https://seniorfriendlyguilford.wordpress.com/2009/11/13/people-who-became-famous-in-old-age/

You are a long way from this tune yet. There is ample chess left for you.

Your chess rating - you can still get it up.

Why do you ask, grasshopper?

If you define reasonably good as Master strength. Yes, you're too old. 

If you define reasonably good as a highly regarded class player, no, you're fine. 

Third paragraph in Mr. Shapiro's obit noted that he had a chess career that spanned 8 decades, he died at age 92. That means he was playing tournament chess at age 12. 

12 is a lot younger than 25.

 The age at which one attains a title is a vastly different question from the age at which one learns the game. They aren't similar. 

While there are no chess specific studies on the topic, presuming that chess is similar to other pattern matching human activities such as language and mathematics, there is absolutely no reason to believe that anyone learning the game after their brains have stopped forming are likely to become extremely proficient. 

I have yet to find signficant examples of players starting learning the game in their late-20s and achieving a master title. 

Even if one or two such (modern) examples exist (and finding even that little is appearantly asking alot), there are tens thousands of people who do start late in life, try hard, and do not achieve such a title.

Given everything we know about the development of chess talent, there is no reason to suspect that any particular person can learn the game past mid-to-late-20s and achieve master level play. But there's plenty of examples of such people becoming strong class A players or even Experts. 

After your first date?

It's never too late for now

Staunton was apparently twenty-six when he took a serious interest in chess. He said that at that time the strongest players he saw in London, Saint-Amant and George Walker, could easily have given him rook odds.^[1] 

^ ^{a b c d e f g h i j k l m n o p q r s t u v w x y z aa ab ac ad ae af ag ah ai} Murray, H.J.R. (November 1908). "Howard Staunton: part I". British Chess Magazine. Archived from the original on December 8, 2007. Retrieved 2008-06-19.and Murray, H.J.R. (November 1908). "Howard Staunton: part II". British Chess Magazine. Archived from the original on December 8, 2007. Retrieved 2008-06-19.

 https://en.wikipedia.org/wiki/Howard_Staunton#1836.E2.80.9342.2C_first_steps_in_chess

In general I guess Kingpatzer is correct in presuming most of us won't become masters starting out at age 20, but I think it has a lot more to do with time constraints than brain development. By 20 most of us are either pre-occupied with a job and/or school and/or family. And I doubt if there are that many players really interested in becoming a master esp if they figure out that Edison's formula usually applies, ie genius consists of 1% inspiration and 99% perspiration.

10 years before death.