I'd be very cautious in assuming anything about their identity merely from the clothes they like or what their interests are.
But, knowing the process itself, I would probably get them started down the road of talking to psychologists, hormones and the like as early as possible into puberty, if it were clear they were serious about it.
Surgery can certainly wait until they are adults, but honestly, if you force them to go through puberty in the wrong body you are potentially causing them permanent physical harm that will make the rest of their adult life much more difficult than it needed to be.
That's not to say you should let them jump blindly into stuff like that, but don't go around forcing them to wait until they're 18. That's cruel, and far too late in some ways...
If there's doubt, puberty blockers are a good starting point. They have reversible effects, and cause no real permanent harm regardless of what the child may decide when they are older.
Surgery isn't time critical, but forcing someone to go through puberty is vicious and cruel, and has irreversible consequences that can make their whole life vastly more difficult than simply being transgender would already be for them.
It's as cruel as surgically altering your child against their wishes, honestly...
So... Think very carefully about it.
If there's doubt, don't rush into anything permanent either way (hormones, whether that produced naturally by the body, or given artificially have quite a few permanent consequences, so the best thing to do in case of doubt is block ALL of it, both what their own bodies would do AND artificial sources. - If it's ambiguous what they really want, this is the safest and least cruel alternative that is least reliant on knowing for sure what they actually want, and makes it possible for them to make the permanent decisions when they are adults, or at least older teens, without having to deal with reversing permanent damage to their bodies somehow. Which is difficult and expensive.)
But anyway, having said all that, it's not as simple as what kind of toys they like, and what their interests are. Reading too much into that leads into really dangerous territory that completely obscures the reality of the issue.
You are not transgender simply because of that. Sorry. There's something more fundamental, and if you have a child who is having issues, it's this fundamental aspect of identity that you need to be watching for, not the superficial nonsense of gender stereotypes...