Deciding to start trying to conceive a child is certainly a grand journey to embark on. My advice about becoming a parent would be to prepare oneself on other levels besides the physical level - I'd suggest to prepare yourself spiritually, mentally and emotionally. We are human beings made up of much more than just our physical bodies; God created us with emotions, thoughts, and a soul. All parts of us should be prepared to become a parent.
In addition to those direct elements to embark on the journey towards parenthood, I'd suggest greatly to prepare financially. Children are NOT cheap, not by far. If you are in such large debt to cause you to hesitate to get married, having children will not decrease the amount of time you get out of debt. Instead, you will be less likely to get out of debt sooner, and therefore more likely to push off getting married as it will take longer to get out of your debt.
Its very rare for a not married couple to try to have a child intentionally.
It is rare for a Christian couple to try to conceive a child out of wedlock, I will agree on that point. However, in our current anti-religion culture and a marriage is nothing more than a piece of paper mentality, it is actually quite common for couples who are not married to try to conceive a child. Mostly the norm around the area I was raised was have children first, get married later.
I hate seeing young kids with old parents that can't keep up with them, its just really sad. And I refuse to be one of them.
While an agile 21 year old will be able to "keep up" with her child, not all 21 year olds can or will do that. In addition, because one "older" person you know can't keep up, does not mean all "older" people can't keep up with children (logical fallacy: hasty generalization). "Keeping up" with young children is a matter of physical fitness. How fit and healthy a person is will determine how easily one can "keep up". Twenty one year olds generally have a faster metabolism and are able to keep fit easier than a 31 year old; which means that the 31 year old will have to work a bit harder in staying fit. If your family member who is older can't keep up, that is more likely a sign of physical inactivity, or lack of fitness more so than a sign of old age causing one to lack vivacity.