Two parts of your credit score are total accounts and average age of accounts . Generally you want to open multiple cards as early as possible so you can start gaining credit age on them. If you haven't had cards in the past you may have to start with lower tier cards or secured cards.

Dave Ramsey is strictly wrong. Like most old people, he has no idea how credit works. If you want to get into the 800s and have lower rates on every mortgage, and get access to better cash back cards (which are strictly better than cash/debit for purchases), you have to play credit like a game.