Jordan Spieth is golf's golden boy. The reigning PGA Rookie of the Year became the first teenager to win on Tour since the Great Depression last year, and he was the youngest American to make a Presidents Cup roster ever. With nearly $7 million in career earnings, Spieth's success has already translated into a fat bank account. But is he set to make even more money from endorsements?

The latest deal

Earlier this week, he signed a sponsorship deal with AT&T (NYSE:T), the Dallas Morning News reports. Although the value of the agreement wasn't revealed, the company did say -- expectedly -- that it will last for multiple years.

With AT&T, Spieth joins an experienced PGA Tour marketer. It received heavy exposure at this year's Masters, and the company has naming rights to the annual Pebble Beach National Pro-Am. Next year, AT&T will also become the title sponsor of the Byron Nelson Championship.

The Under Armour connection

Under Armour (NYSE:UAA), though, takes the cake in terms of visibility among Spieth's sponsors. The apparel giant signed him back in January 2013 when he was ranked 809th in the world, a whopping 801 spots lower than he is now. ESPN's Darren Rovell says, at that time Under Armour expressed "wanting to sign Spieth for 25 years."

While the deal probably isn't longer than a decade -- that's the estimated length of Rory McIlroy's Nike (NYSE:NKE) contract, for example -- Under Armour still has to be very happy. Spieth is the first golfer in the company's history to don its gear from top to bottom.

At this year's Masters, where Spieth finished tied for second and led heading into Sunday, he displayed the "UA" logo on his pants, shirt, hat, and belt -- close to 10 in total.

Adding it all up

According to Forbes, Spieth also has endorsement deals with Berkshire Hathaway (NYSE: BRK-A) subsidiary NetJets, Rolex, Perfect Sense Digital, BioSteel Sports Supplements, and of course, Titleist, which supplies his equipment.

Golf Digest reports Spieth made $4.5 million from endorsements last year. On the course, he made about $4.6 million.

Income of World Golf Top 10 (in millions) Off-Course On-Course Off/On Ratio 1. Tiger Woods $71.0 $12.1 5.9 2. Adam Scott $7.6 $8.0 0.9 3. Henrik Stenson $2.9 $18.5 0.2 4. Bubba Watson $4.2 $2.2 1.9 5. Matt Kuchar $3.7 $7.1 0.5 6. Jason Day $2.5 $5.2 0.5 7. Sergio Garcia $8.5 $3.4 2.5 8. Jordan Spieth $4.5 $4.6 *1.0 9. Justin Rose $5.5 $6.2 0.9 10. Rory McIlroy $18.0 $2.6 6.9

As seen above, the ratio between types of income reveals a lot. McIlroy and Woods, for example, make about six times as much money off the course as they do on it. Elite fan favorites like Bubba Watson and Sergio Garcia make about twice as much off the course. The less marketable members of the world top 10, like Jason Day and Matt Kuchar, tend to outplay their endorsements. In Spieth's case, he appears to be in a middle.

Looking ahead

So how much money will he make off the course this season? That depends, but with his new AT&T deal, the total will be above the $4.5 million he made in 2013. Assuming Spieth continues to play well, his ceiling in the short term -- in terms of endorsement potential -- likely could mimic a Sergio Garcia or an Adam Scott. Both made upper seven figures from sponsors last year, but they're not close to Tiger Woods or Rory McIlroy territory.

Ultimately, Spieth isn't either. If he starts winning majors that could change, but something tells me that's O.K. with the 20-year old from Texas. He's 13 years younger than the rest of the world's top 10, on average. And that -- youth -- is the real reason companies like Under Armour and AT&T are throwing money at him. As long as Spieth continues to play at a very high level, it won't stop any time soon.