By prioritising it higher than any other non-essential cost you have. I know people who have saved up for 6 month round the world trips while working in various menial low paying jobs. The question is, like for most things, is how badly do you want it.
Of course it helps if you don't have a family to support.
..or I could work hard, get my startup to exit, and travel as much as I want for the many years in life afterward. Why does travel have to be done when I'm in my 20s/30s/etc.?
It doesn't, but it depends on what you want out of life. If you want to have a successful business, traveling all the time isn't for you. If you want to see the world though, putting it off until later isn't going to help you any. The longer you put of traveling, the less you'll be able to do, and the more entrenched you will be in your work.
Travel isn't, in my opinion, something you should 'have done', it's something you should be doing. I'm not a big fan of the take a year out, travel the world, then tick it off your list. Travel anything from a week to a month every year until you're too old/sick to get out of bed. My 86 year old grandmother recently flew across the Atlantic to visit family.
The world is not a static place. That thing you really want to see might not exist a few years from now. You can visit the same city a half dozen times and have a half dozen different experiences of the place because the world has changed it between visits. I'd rather take a few long weekends every year than save up for one big trip sometime a decade from now, but that's just me.
Because the human body ages, and traveling involves physical stamina. When I travel, I often end up walking around multiple hours a day for a week or two. This is getting more difficult as I get older. I can imagine that when I am in my 50s or 60s, those tour buses will start looking mighty tempting.
Its relatively cheap to travel abroad, especially if you're willing to live simply while you're there. If you plan significantly far in advance, you can usually find airfare for $500-750 round trip, or cheaper, depending on where you want to go. Then if you plan it out, you can usually stay wherever you are for <$100 a day.
I'm not sure about where you are, but even someone in the US, working full time at McDonalds, making $6 an hour, could afford a week long trip every year by putting away 10% of each paycheck. $1200 is (roughly) enough for a week long trip to most of the non-tourist packed places, and you can save that up by putting away a little more than $20 a week. Just making lunch at home rather than eating out (or buying a pre-made lunch) can save you almost that much. Its just a matter (like he said) of priorities. That isn't to say that some people can't afford it. My example assumed someone with no family, medical problems, or other large expenses, but it was also a fairly low end of the pay scale (for the US).
$100 a day!? That's not living simply - that's staying in hotels and eating in restaurants. I'm an active CouchSurfing host and I've met many travellers who get by on $100 per month. And that's in Europe. You can stretch that out for 3 months in lots of parts of Africa or SE Asia.
CouchSurfing and equivalents can drastically cut your accommodation expenses. Hitch-hiking or cycling give you free travel. Even travelling by bus can be remarkably cheap in most countries if you go the slow way (which also lets you see much more of the country that just the capital) - you can often go 50-100km for a couple of euros. Your primary cost is food, and if you've learned to cook even a few basic meals, you can easily get by on a couple of euros a day.
Travel is basically free if you're doing it for any length of time.
In Africa, Southeast Asia, or Central America, you can live on $5-$20/day depending on where you go. Hitching through Malawi will run you $3/day tops. Assuming you've ditched your rent/car payments/etc. back home, you're going to be living an order of magnitude cheaper than you would if you simply stayed in the USA.
For short trips, your flight will be the dominant cost. For long trips, you're just not going to spend that much.
If you can put away a measly ten grand, you can stay gone for a long time. How much did you make this year? More to the point, why didn't you manage to save at least $10k of it?
