I thought I'd share my experience with what I meet the most as a 1 e4 e5 player.

From 28 tournament games over the last three-and-a-bit years where I had Black against 1 e4 (excluding the one game where I went for 1...c5 and got crushed); 

26 of the 28 players (who total averaged ~2034 national rating) chose 2 Nf3 (The remaining two games saw 2 f4 and 2 Bc4, a draw and a loss). From these 26, half played the Spanish, and half some other form of Open Game (Italian, Scotch or Four Knights). 7 of the aforementioned Spanish games went down the traditional absolute main line (i.e. 3...a6 4 Ba4 Nf6 5 0-0 Be7 6 Re1 b5 7 Bb3, and now either 7...0-0 or 7...d6), while the other 6 deviated in some way (4 Bxc6, 5 d3 or 5 Qe2).

Good luck in your 1 e4 e5 adventures!

an ordinary chessplayer wrote on 08/14/18 at 16:47:36:

You need to know how to deal with the King's Gambit, and if you play something like the 2N against the Italian, probably some lines against 4.Ng5, as well as a baseline against the Scotch, as it's rather weird/concrete. The Scotch Gambit comes to mind as well, but now I'm already in "not sure if needed" territory.
Beyond that, most things are either extremely rare, or one can deal with them over the board. My FIDE is 2000, I don't feel like I know a whole lot, and I still out-opening all my opponents in every game.. 
If you find your opponent plays X, you can always prep for it specifically, and if you lose to Z, you will learn it after the game, and done. I lost an online Blitz game to the Danish where I got slaughtered, then I learned that one antidote line with Bb4 Bg4 Bf3: and Qc4 which takes all the fun out for White, and since then I've won every game against it (not that I've fought it very often since then). Not exactly an unsurmountable wall..

I don't know, maybe I just live in a theory-free-zone, but over here people generally are out of book on move 5 or so with either colour, so it doesn't matter too much if you're out of book earlier than them.

Here's a sample start to a game I had against a 2250 earlier this year:
 
We were both out of book on move 7, I just played what I deemed to be natural moves, and I was risk-free better on move 14.

Of course, sometimes you run into some weird sideline that your opponent knows, naturally once in a blue moon you will meet a theory monster that just bolks out the first 20 moves in .1 seconds, etc whatever.. but that can happen in literally every opening.
At least in the Open Games you can be sure that your position is fundamentally sound, that "the solution" will probably be some kind of natural move, and that if you find the correct answer to the White weirdness, you will likely end up not just equal but even better! 
By contrast, eg against the CaroKann I play the Advance variation with 4.h4. If Black doesn't know this, he just loses. If he does know it, it's still a good opening where I am fighting for an advantage in critical lines..

If you forget a move in the Giuoco Piano, you can play ..a6, or ..h6, or ..d6, or castle, or ..Be6, or remaneuver the Knight from c6, or ..Re8, or ..Kh8, or one of five billion other natural moves. You will probably not find the right answer to whatever critical line your opponent may uncork (ha-ha, that already doesn't really happen), but it will be vaguely playable.
If you forget a move in the Austrian, you can just resign. White will steamroll you, goodnight.

Related: From my personal experience, almost nobody plays 1. ..e5 in the 1700-2200 range. It's like 50% Sicilians, 20% Caro/French, 20% "personal unique whatever" lines like Alekhine, and then 10% 1. ..e5 out of which the majority again chooses Petroff/Philidor/etc .. which in turn means that White players aren't really well versed in their 1.e4 e5 repertoires either. 
Of course, depending on your scene, that may differ. Over here, I've played 1.e4 seriously again for ~2 years, and in ~100 games total (so roughly 50 White games) I managed to get 2 Ruy Lopezes on the board, one where Black went for that Norwegian(?) thing with a super early ..Na5, and one where my opponent mucked up the moveorder rightaway (going ..b5 prior to ..Be7). 
If someone played a halfway mainline Breyer or such against me, I'd just be out of book after h3, as I've never seen any need to learn it whatsoever. Most people avoid mainlines like the plague.

Bottom line is, "amount of theory" only matters when your opponent knows it. And from my own experience, people generally don't know jack, so you don't have to know a whole lot to be able to handle them either..

Having never been 61, I am not in a position where I can give advice on how your age should affect your opening choices.

However, you clearly are interested in the open games. Isn't that reason enough to learn them?

I don't believe I would ever discourage anyone from taking up the open games as black. Apart from general chess education, it would also make following top level chess more enjoyable.

an ordinary chessplayer wrote on 08/14/18 at 16:47:36:

I was thinking of doing just that at my local club! I already know of one King's Gambit player…

IsaVulpes wrote on 08/14/18 at 15:57:55:

I was thinking of the Closed Ruy Lopez. I already bought Marin's QC books and ordered "Play the Open Games As Black" from Emms and was thinking of buying "The Ruy Lopez: A Guide for Black" from Johnsen & Johannessen.

IsaVulpes wrote on 08/14/18 at 15:57:55:

I forgot to mention that I played the Dragon in the first years of my "carriere" and play more "slowly" in the last years. I mostly used the Modern move-order to dodge the Austrian.

Hello, I am a 61 year old player, rated 1900+ national and 2000+ FIDE. For about 30+ years I have played the Sicilian (Dragon) with Black, than about 15 years the Caro-Kann and now about 3 years the Scandinavian and in between some Pirc / Modern's. As a beginner I never properly learned 1.e4 with White and certainly not 1.e4 e5 with Black. I feel like I missed something in my chess education and was thinking of taking up 1.e4 e5 with Black. But I am a bit put off by the enormous amount of theory and the more tactical nature of the open games. I am more inclined to a slow built up in the opening. Also my energy, memory and concentration are (a bit) less than when I was younger. What do you think, is it worth the effort or am I better off with my current repertoire (mostly Scandinavian), also considering that I am not that young anymore?