I can kind of see that argument, but remember that there are also headshots on the site, so it's unlikely that someone will pick a 40 year old to play a teenager, or some similar age mix-up. At most, I think that it should be left to each actor whether to display their age or not, or perhaps even give a range of ages they can play. I'm not sure why imdb dragged their heels on this.
It's not really pointless if you think about the process. A producer is considering a bunch of unknown actors that they've seen on imdb. They see the headshots, credits, etc and determine that the actors seem suitable for the part. Do they have the time to go digging around google to find the age of each actor? Especially if they're unknown, the information is probably even harder to find. They will most likely just forget about real age, move along to the next phase and invite them for an audition, or whatever.