How many villagers can I support if I have coconuts, an algae-infested ocean, and am currently working on a dam? My youngest villager is 10.

About 10. Maybe little more.

I'd say up to a maximum of about 15. I'd make sure you have at least a couple more kids soon, otherwise you'll have a gap when you've no-one to pick up collectibles and mushrooms.

It's better to play in ½ speed, villagers eat less. For example, I began a new tribe yesterday (in normal difficulty because I'm bored to run after food as in my previous tribes):
With 7 people, when the algae came I was able to buy the level 2 of Engineering and I put 5 villagers on building the dam and 2 children (and 726 points of food from fishing, I didn't have coconuts). I spend my time with children picking mushrooms and collectibles. 2 hours later, the dam was build and I wait 1 hour and ½ for new crops. When crops arrived, I had around 880 points in the bin without coconuts.
But it was in normal difficulty, ½ speed and looking for mushrooms all the time! In hard difficulty, you need the coconuts and try to don't miss a mushroom!! It is possible with 10 but no many more. (and in 1/2 speed)

Sorry! I misread the original post. I thought you said you had completed the dam! Cut my original figure down to ten at the maximum in that case. Also, put all your available villagers on building the dam. The sooner that's built the better. Don't even think of expanding the tribe any more until the dam is completed.

Oh yes! but:

May be to day he is 14,so take care to make an other one! You need that for picking up mushrooms!

Agreed, laurence, but ...

With as many villagers as possible working on the dam, it shouldn't take that long. Once the dam is built they can relax and perhaps celebrate by having a couple of babies!

Absolutely!

a couple of babies it's better they can play tag!!
My 5 villagers have built the dam in 2 hours, I'm proud