It's encouraging and depressing at the same time to see everyone having such responsible thoughts on the issue. It really is a problem that all the wrong people are having the most children.
I have two kids and plan to keep it that way. I think that if you're going to have kids, it's selfish to have any more or less than two. No offense to any here who were raised as such, but only children are in too much danger of growing up self-centered. Siblings contribute immensely to a child's emotional maturity. You are doing them and the world a disservice by only having one. Any more than two, and you're increasing global population, which I also believe to be selfish.
Also, I think it's incredibly beneficial to have them close together in age. It may be a bit rougher on you in the short term, and they might get annoyed about being unable to have separate social lives outside of the home. But they'll also be able to do a lot more things together and have a much richer shared history that will lead to a deeper bond later in life. My siblings were 3, 6, and 10 years younger than me, and they never wanted to do anything with me. Most of my interaction with them, especially the last two, were in a babysitting role.
There will never be a perfect time to have kids. You will always be able to think of some unfulfilled ambition or further preparation as an excuse to put it off. It also gets harder as you get older. Kids take a ton of energy. You'll miss many nights of sleep. You'll be a lot more capable of handling this in your 20s than in your 30s. You'll also get do to more things with them and be around for more of their lifetime. My parents had me when they were 20, and I love that my dad was able to play full contact sports with me in my teen years. Now that I'm 28, my parents are still healthy enough that we're looking for opportunities to go on a couple last adventures together, all as adults. One thing we want to do is a motorcycle trip from Indiana to Alaska. Ten years from now, I doubt they'll have the fortitude. My dad's already had a minor stroke and heart problems. It wouldn't have been the same had I been born a few years later.
I had my first kid at 21. It was an accident, and I wish it had happened under better circumstances. I've lost a ton of opportunities. Do wait for a good relationship and some financial stability. I mean stability, not wealth. Don't set a six-figure income as your pre-requisite goal. Still, it's great to enjoy my youth together with and invest in my kids. I might fantasize about life without kids or even if I had just waited a few years, but I really have few regrets.
It's great to see so many people talking about adoption.
Finally, if you choose not to have any kids, please PLEASE go out and spend time with some anyway. Hang out with friends and family who have them. Babysit for them. Get at least a taste of the experience of parenting. I don't think everyone has to enjoy being around children or go out of their way to learn to like them. However, I do think exposure to children is an essential component of personal growth as an adult. With so many people choosing to avoid kids or waiting until their 30's, I feel there is an epidemic of spoiled, bratty adults among my generation. The majority of people my age without children seem to have no patience for anyone but themselves and no appreciation (and thus no compassion) for the needs and difficulties of parents. They flip the fuck out at the slightest child-like disturbance of their perfect adult-only world, and harbor a poisonous hatred for parents whose responsibilities get in the way of other adult stuff, such as being unable to go out and do things spontaneously or requiring extra time off from work ("OMG parents get unfair special privileges!").
Ultimately, I do not think children are about contributing to the gene pool or continuation of the species or being lovable and fun or filling some void in your life or some investment that will pay itself back later on. What they have to offer more than anything else is personal growth. I personally don't think there is any magical life-changing moment when you see your kid for the first time or unbreakable spiritual bond or deeply buried instincts that get awakened or anything like that. Parenting is confusing, terrifying, draining, frustrating, humbling, and in every way a learning experience that you cannot get otherwise. You have a human being who depends on you 100% for absolutely everything all day every day. You have to provide for them and teach them how to live. I cannot emphasize enough how much of a personal growth experience this is. You learn so much about responsibility, compassion, and the nature of humanity and life in general by dedicating yourself to the growth of another human being from something that squirms in the palm of your hand to an adult who can kick your middle-aged ass if you deserve it. The only way to elevate oneself is by rising to a challenge, and parenting is one of the greatest challenges there is.