John Scalzi explains why most "new novelists" are in their thirties or older. Amen — my first book came out when I was 30, and I was considered a young turk:
Finding an agent is a slog. One has to query the agent, wait to see if the query is accepted, and then if it is sample chapters and an outline go out in the mail. Then more waiting to see if the agent asks for more. If he or she does, it's time to send the whole manuscript and then wait again to see if he or she thinks the writer is worth their time to represent. At any point the agent can say "no," at which point our budding novelist will have to start over again.
But if the agent says "yes," then comes the part where he or she starts schlepping the novel to publishers. Presuming the agent gets a publishing house interested in looking at the manuscript, it could be weeks or even months before there's response, either positive or negative. If it's the latter, it's on to the next publisher.
The second path is the Path of the Slush Pile. This gets the work out there quicker but fewer publishers still accept unagented manuscripts, and as you might guess from the name "slush pile," the rate at which editors work through the slush pile is pretty slow. Baen Books, which accepts unagented manuscripts, lists their response time as nine to twelve months: Yes, you could make a baby (if you can make a baby) before our poor theoretical writer here would hear back about their literary child. And if at the end of those nine months to a year Baen (or whomever) said no, the poor writer have to start all over again.