20-Year Prospect
Criminals have been in existence since the first caveman shoplifted the first stone axe at his local NeanderthalMart, and they'll continue to exist right up until climate change or contagion or aliens snuff us all out. This means we can probably count on Uncle Sam needing ATF agents to investigate violations of federal law for the foreseeable future.
The number of ATF agents in existence, however, is something you might have to worry about.
See, there's this lovely little body of elected individuals called Congress, and Congress sometimes likes to do things like not fully fund the government, just for giggles. For the ATF, budget cuts mean personnel cuts: The Bureau can't recruit and hire as many agents as it needs to function effectively. The number of ATF agents has been dropping since 2010, from 2,562 to 2,402 agents employed in 2013 (source).
In other words, while ATF agents will still be around about the time you're ready to embark on a mid-life crisis, this job is already pretty hard to get and could become even more unobtainable in the future.