I started when I was 25, so I can't speak to the 25 by 25. Here's what my first three years in real estate looked like.
Door number one duplex 3% down. 75k = $2250 (I lived in one side, so I'm not counting my door)
Door number two $20k = $2k down + backroom bank loan (sort of like a personal loan)
Doors 3,4,5,and 6 $110k = $0 down it was in the time of creative loans and financing.
So, that's 6 doors in two or three years for cash at close of $4,250
Now, some people will do wrap around loans, land contracts, and other means of creative financing. - Which you will have to, as you can only hold 10 loans (I think) loans through Fanny/Freddie.
The next two purchases happened within a few months of each other, the houses were next door. While I haven't shown you a path from 0 to 25 in a few years, you should be able to see that even my bungling around happened at a quick clip, and not for a ton down.
Anyway, I am sure somebody more savvy than myself will draw this all out.