There's always going to be a cut-off with all important decisions, though. Even if you somehow made a law that newborn babies could vote - and miraculously they were actually able to do so - there would be more babies born the day after the election or referendum who would have missed the boat.
In my opinion - and others will disagree - it isn't really the case that each individual gets a say for themselves, but rather that the country (or home nation/constituency/city etc.) as a whole makes a decision on a pre-determined day and this is done by asking each adult (or soon to be everybody over 16) for their input. It may sound like the same thing, but I see a subtle nuance.
Yes, if there's a groundswell of support for a reconsideration and there's been a sea-change in the political atmosphere across the land, laws can be voted on to be changed; but practically, we have to trust that the demographic of adults (or nearly-adults) made a representative and enduring decision on behalf of everybody.
What about the old laws that have been in force for decades or even centuries? I'm nearly 50 and I've never been given a chance (nor will I probably ever get a chance) to have my say by voting on the death penalty or lobbying my MP (should I wish to change it, which I personally happen not to) - and countless others? In fact, look at the outcry over the fact that people aged under 60 were given a vote as to whether or not we stayed in the EU, when only those aged over 60 had previously had a chance to vote to enter - and even then it was a decision about a relatively simple trade agreement and nothing really resembling what the EU had developed into.
What about laws that are voted on by all MPs at one point in time, on behalf of the whole country: ones that can't easily or realistically be repealed or undone at a later date? Why don't we make all laws to be valid only until the next election, and then make all of the newly-elected MPs vote on every single existing law to see whether it is kept, scrapped or amended? In fact, you could ask why people who turn 18 (soon to be 16) may have to wait five years before they can have their vote in a general election. That's nearly a third of your life if you're 16!