Hey Damian, Congrats on getting motivated to do this at such a young age. How much to save really depends on how much the price is of the place you are going to buy. Just starting out, a duplex is the best way to go, but that is not considered commercial, it's still residential (anything 1-4 units is considered residential) and you would be getting a conventional loan. Since you will be living in the property in one of the units, you would only need 3.5% down, so that equates to 3.5k for every 100k in price (for example, a 200K duplex is going to need 7k down), plus depending on your mortgage lender and what you can negotiate the Seller to pay of the closing costs, I would estimate another $3300-3700 for that to be safe.
What is the best age you ask? I'll answer that with an old Chinese proverb you'll hear thrown around the investing world (I hope I don't butcher it):
"The best time to plant a tree was 20 years ago, the next best time is today" Start ASAP and Good luck!