How old is the effective altruism movement? It depends when you count from, and it's hard to pick any specific day or event because it coalesced out of a lot of different strands. Plus, since this is a movement and not just an idea, it matters when it started gathering people which is naturally very fuzzy. Roughly, I'd describe the progression as something like:
At the beginning of 2008 there wasn't an EA movement yet, while by 2012 there was one. There wasn't an instantaneous change.
Why this particular curve? Here are some historical points that anchor it to events:
In March 2012, I wrote "there's a young movement that combines the idea of a duty to help others with the idea that you should maximize the impact of your actions," as part of trying to figure out what we should call EA. I'd say it 100% existed by this point, and I wasn't "calling it"—it existed before here.
In November 2011, 80,000 Hours launched. It was mostly a way of spreading ideas that were already popular in the Oxford community and online, so I'd also say the EA movement existed before this. It's a bit less clear than when you have people literally calling it a movement, though.
In December 2010, Roko Mijic ran a contest on LessWrong for the best explanation of EA, which Scott Alexander won with his essay, Efficient Charity: Do Unto Others.... Jonah Sinick also wrote a detailed entry, and Mass_Driver contributed one as well. This feels very movementy to me: a bunch of people working together to put some shared ideas into a form where they'll be able to spread better.
In November 2009, the first 30 people joined Giving What We Can. A group of people pledging to do something seems like part of a movement. At the time GWWC was specific to global poverty, however, so I wouldn't fully count this.
Over the course of 2009, discussion on the Felicifia forum (archive) started to feel like an early EA community. For example, see thread on charity choice and the applied ethics and philanthropy boards.
In December 2006, the first GiveWell blog post comes out. Looking back at it, and their other early posts, you can tell that Holden and Elie had a lot of what has since become EA in mind. This also goes for a lot of other early writing like Nick Bostrom's 2003 post on astronomical waste, Anand's 2003 SL4 "EffectiveAltruism" wiki entry, Brian Tomasik's 2006 post on earning to give, and Eliezer Yudkowsky's 2007 post on scope insensitivity (that coincidentally used the term "effective altruist" years before it was settled on). These are helpful for dating the ideas, but I'm looking for when the ideas started gathering people and they seem to predate most of that.
One thing I'm not putting much weight on here is the founding dates of organizations. While many EA orgs have played a large role in the EA movement, the oldest became EA over time instead of starting out that way. For example, MIRI was founded in ~2000 as SIAI, but was initially wasn't something you'd recognize as EA and didn't have an associated community. It did grow in both of these directions over time, but to the extent that this helps us figure out the timeline of the movement we need to look at those later developments. Similarly, ACE could (but doesn't!) claim to have been founded in 2010 as JFA, but before merging with 80k's EAA it had a pretty different focus.