>Am I too old to learn it? I mean, I don't want to be a pro or something, I just want to play it for fun, as a hobby.
Not at all.
>How do I begin with it?
Buy a guitar(probably the hardest part for a brohno, but, oh well), amp, picks, strings(the stock ones are usually bad and shouldn't last much). Get Guitar Pro/Powertab, go to sites like 911tabs.com and search for some songs you like and you think must be easy.
There's tons of video tutorials on youtube too, and websites with free resources for learning, google is your friend.
>Is it possible to learn it just by reading tabs and listening to the music?
Depends how exigent you are regarding your performance, but I wouldn't say it's possible anyway. Watching videos(supposing you don't want to get a teacher) and doing specific exercises are strongly recommended, as it's quite easy to develop bad habits with really basic things such as merely picking otherwise.
Now if you mean theoretical knowledge, you don't really need much, if any. But it's really useful, and it's quite pleasant to be able to understand what you're playing.

>Am I too old to learn it?
Unless you have some wild case of precocious arthrirts: not at all.

>I want to learn it by myself
If you say so because you don't want to go out and/or interact with people, I understand completely: that's why I'm trying right now to learn piano that way; so I would say that it'll make the task a lot harder, but not impossible. For guitar, I did took a few classes on highschool, on wich I learned the very basics for classical finger picking style; things like how to hold it, tune it up, what finger goes where and how to exercize them, some basic chords; but the rest I learnt it by myself, basically finding tabs and chords online for the songs I liked, each one giving me new rythm and harmony sources to add to my experience; as the above Anon said, now we've got as well a great resource: tutorial videos at Youtube et al.; for electric guitar, you'd unavoidably have to learn some scales as well (check: http://www.chordbook.com/guitarscales.php )...

I'm >>75 and >>1918, btw.

Post edited on 21st Jul 2012, 1:15am

These should be useful, at some point:

http://www.youtube.com/watch?v=DmJQ-lgMoEA
http://www.youtube.com/watch?v=mg6MLUnGG3U
http://www.youtube.com/watch?v=SDbx6Nfc_no
http://www.youtube.com/watch?v=L6NAQgZGm9Q

Post edited on 21st Jul 2012, 1:36am