A lot of the responses here are either on the side of being fully monetized when they start, while others don't think you should have ads at all. Personally I think there's a middle ground.
Can you have too many ads on your site when you start, to the point where it makes your site feel like it's about nothing but ads/money? Sure. But I also think there's something to be said for having some ads, especially adsense ads, on your site from the start. I had ads on my site from the start, and while I didn't make a ton from them right away, I made enough to be able to pay for my domain name and hosting.
Also, having them there right away meant that users were used to seeing them there from the start – and there was no point in the future where they were feeling like I "sold out" because I added ads to my pages.
I have to tell you that I have only heard complaints about the ads on my site once or twice in 3 years. Considering I've had over 3 million page views, that's not that bad. Were there others that didn't complain, but stopped reading because of ads? Certainly, but I don't think they're the majority.
I don't think there's anything wrong with monetizing your content – even if you're mainly doing it for fun. Your writing is worth something, and why not be compensated for your hard work and worthwhile/useful content?
Having adsense ads on your site can also be useful. When hooked up with a google analytics account it can help you to figure out what articles are getting the highest paid clicks, which topics pay the best, and also what types of topics people are finding the most useful. That in turn can help you to improve your content to make it more useful for your readers.
While you can probably tell by now that I'm more on the side of including ads from the outset, i should also note that I don't think you should add a ton of ads – adsense, CPM ads, or text link ads to your page. As far as adsense I try to keep it to one or 2 ad units per page, sometimes just 1. Having more ads doesn't necessarily mean you'll make more money. In some cases I've found one well placed ad unit will outperform 3 poorly placed ones.
So, I come down in favor of monetizing from the start, but in moderation. Don't go overboard, and make sure you know the risks of certain types of monetization like text link ads..
Best of luck!