(Warning: The following theoretically contains SPOILERS for Captain America: Civil War. That being said, if you've already seen a trailer or a TV spot for the movie that features a certain friendly neighborhood wallcrawler, then you're pretty much safe to read on. All the same, proceed with whatever level of caution your Spider-sense deems wise, and all that...)
Now, for all that we as a collective geek culture love Robert Downey Jr's portrayal of Iron Man, there's still a wide-ranging willingness to call the Marvel Cinematic Universe's most famous hero out on his flaws. He is, after all, an ex arms-dealing egomaniac with trust issues and a habit of accidentally creating sentient AI with a penchant for global extermination.
So, when it emerged that Captain America: Civil War was set to feature Iron Man recruiting a certain Peter 'Spider-Man' Parker to fight alongside him, despite Parker being all of fifteen years old, and therefore prohibited from fighting in a war by any number of UN conventions, it's fair to say that a whole lot of eyebrows were raised.
As it turns out, though...
There's a Surprisingly Solid Reasoning Behind Tony Stark's Recruitment of a Teenage Spider-Man
Specifically, as Captain America: Civil War's co-director Joe Russo recently put it during a press conference, Iron Man may just have been thinking about the greater good:
"Look, there’s a certain narcissism to the character and Tony doesn’t want to lose this fight, and at the same time I think he also sees Spider-Man as the greatest living non-lethal weapon. If you’re going out to capture a bunch of people who you don’t necessarily want to hurt, you couldn’t ask for a better character than Spider-Man to take with you."
Which, crucially, doesn't absolve Stark of his responsibility to, y'know, not recruit minors to fight his battles for him, but does suggest that his reasoning was based at least as much on the prevention of harm as on his desire to win at any costs.
What's more, Russo went on to argue, Stark is well aware of exactly how strong - and thus unlikely to get badly injured - Spidey is...and has put plans in place to keep him safe:
(The following quote is a little more SPOILER-ific, and as such is hidden behind some delightfully blurry SPOILER-protection, just in case you don't want to know any minor details from Captain America: Civil War...)
"[Stark] does show up and illustrate that Spider-Man, he knows how powerful he is...In a video we see Spider-Man stopping a car moving 40 miles an hour, where he catches it and puts it back on the ground, so I think he believes that he’s taking with him perhaps one of the most powerful characters in the Marvel universe, and I think he feels like the kid will be well-protected under his tutelage. You also find out in that sequence, when things go wrong the kid says, ‘What do I do?’ and Tony says, ‘Keep your distance, web ’em up.’ So he’s obviously mentored the kid for what’s about to go down."
Which, once again, isn't exactly an entirely reasonable justification, but does at least soften Stark's emerging edges.