|Bonds's home run record will likely stand for a long time (NPR)|
Alex Rodriguez (654)
A-Rod is closest, needing 108 round-trippers to tie Bonds, but that doesn't seem likely to happen for a variety of reasons. First, it's unclear whether Rodriguez will ever play again after missing the entire 2014 season because of his PED suspension. He turns 39 over the summer, and age combined with failing health (88 games per season over the past three) will make it difficult to return to his previous level of his excellence. Plus, nobody has ever hit 108 home runs after turning 39, though Bonds and a few others (Darrell Evans, Carlton Fisk) came close.
Albert Pujols (494)
Pujols is 34 but should have his 500th home run by the end of next month (he's currently sitting on 494, having just passed Fred McGriff and Lou Gehrig). In a vacuum Pujols appears to be in a good spot, since only Rodriguez, Jimmie Foxx and Sammy Sosa hit more home runs through age 33. However, it's highly unlikely that he'll hit the 268 home runs (as many as Brooks Robinson and Joe Morgan had in their entire careers) needed to catch Bonds seeing as how his home run totals have gone down every year since 2009. Bonds and Aaron are the only players that have done what Pujols needs to do, but whereas Aaron got a late kick from Atlanta's launching pad stadium and Bonds, um, got his late kick from something else, Pujols is stuck in a pitcher's park that's only going to take home runs away from him.
Miguel Cabrera (366)
Cabrera's been so good for so long (a decade of sustained excellence) that it's surprising how far he still has to go to catch Bonds. Off the top of my head, I would've guessed that Cabrera was past 400 already, but in reality he still may not be at 400 by the end of this season (he has 366 currently). That means he still needs to hit almost 400 home runs which, given his age (31 on Friday) is going to be virtually impossible. After turning 31, only Bonds and Babe Ruth have hit the number of home runs that Cabrera needs to hit to catch Bonds. Put another way, he needs to hit as many home runs as Joe Carter did in his whole career, which probably isn't going to happen.
Prince Fielder (285)
Cabrera's former teammate has 285 taters through his age 29 season, but that still leaves him almost 500 short of Bonds. With a body type that doesn't age well, Fielder won't have the longevity to approach 700 even if he becomes a full-time DH.
Jay Bruce (166)
Bruce, who just turned 27, had more home runs through his age 26 season than Babe Ruth, Reggie Jackson, Mark McGwire, Manny Ramirez, Lou Gehrig, Barry Bonds, and Frank Thomas. He's helped by the fact that he broke in at 21 and showed power immediately, blasting 21 home runs that year (2008). His home run totals increased each year through 2012, peaking at 34 before dropping slightly to 30 in 2013. It also helps that he plays in a homer-friendly park and has been durable. Bruce is on track for 500-600 home runs, but 700 is going to be a challenge unless he can string together a few 40-50 homer seasons, which may be tough to do given how streaky and strikeout prone he is.
Giancarlo Stanton (121)
Just 24 and with 121 big flies under his belt, Stanton seems to have a good shot. He had the tenth-most home runs through age 23, but he would rank a lot higher on that list if he hadn't missed an average of 40 games per season in his first four years. In 2012, for instance, he slammed 37 home runs in just 123 games, which works out to be a 48-homer pace when projected out over 162 games. Stanton's light-tower power is well-documented and makes him a great candidate to break the record, but for a serious run at Bonds he has to stay healthier. It also wouldn't hurt if he could away from Miami's mammoth outfield dimensions.
Mike Trout (65)
Through age 21, only Mel Ott, Tony Conigliaro, Eddie Mathews, Frank Robinson, and Alex Rodriguez went deep more often than Trout. Including Conigliaro, whose career was tragically cut short by Jack Hamilton's beanball, the group's average home run total is 485.6. Ott and Mathews finished with near identical career totals (511 and 512, respectively) while Robinson reached 586 and Rodriguez is at 654 (and possibly still counting). That puts Trout in some pretty impressive company. Playing half his games in Anaheim hurts, but perhaps he will land a massive contract somewhere more hitter-friendly when his extension runs out.
Bryce Harper (43)
Harper hit 22 bombs as a 19 year-old and 20 more as a 20 year-old. Only Ott and Congiliaro homered more through the same age. Like Stanton, Harper has demonstrated massive power that should only improve as he reaches his prime years, but also like Stanton he must do a better job of staying healthy if he wants the all-time home run record.