I'm 22 years old now and I joined SFI almost 2 month ago, and I tell you this.
I wish I joined when I was 18. It's perfect for that age since you don't need any investment money and it's easy to learn and follow.
18 year old people, usually students, can join SFI and start making an income very fast and they will have it for the rest of their lives. So my opinion is, the earlier the better.