Miyano-sama / Japanese Language / Wa as a topic

Notice: The following example was taken from Movie#9 (Strategy above the depth) - the manga version. You can buy if you like from Amazon through the following link: Click here

Consider the following Example:

じゃ～ん！！
カフスボタン型スビーカー、盗聴機能付きじゃ！！
Tada~!!
These are speakers in the shape of sleeve buttons, with the capability of bugging!!
 

あら、なかなか洒落てるじゃない？
wow, they're very stylish, aren't they?

[Conan]
こりゃいいや！！
They're great!!

[Kogoro] (Taking them from Conan)
おっ！！カフスボタンか！？
Oh!! Sleeve buttons!?

[Kogoro]
ちょうどういい...しばらく借りるぞ！！
Just in the right time... I'll borrow them for a while

[Conan]
でもそれは...！！
But these are...!!

バーロォ！！
こういうのはオレみたいなダンディー男にこそ　ふさわしいんだ!!
Idiot!! Things like this are better suited for Dandy man like me!!

[Kogoro]
オマエみてぇなガキにはまだ早えんだよ...
It is still early for a kid like you (to wear them)...

[Ran]
でもお父さん...
But Dad...

Why did Kogoro use the particle "wa"?

Kogoro could have said 「お前みたいなガキに　まだ早いんだ」, without using the particle "wa". Either way can be interpreted into English in the same way. If "wa" was not necessary, why did Kogoro use it?

The reason is that Kogoro wanted to point out that wearing the sleeves buttons is early for kids but that is not necessarily true for others. "Wa" in this case gives the nuance that it is not early for someone else (that is Kogoro) to wear the sleeves buttons. Therefore, Kogoro is saying in an indirect way that the sleeves buttons belong to him since it's early for Conan to wear them.

The above nuance could not have been conveyed if Kogoro did not use "wa". Whenever you spot the particle "wa" coming after something that does not appear to be subject, then be sure that it is making a contrast with something else.

Is it really common to use "wa" in this way?

From my experience, I can say that "wa" comes most of the time after the subject. To give you a better picture, I used Detective Conan manga to search for a live example about this kind of "wa", and I only found the above one after 38 pages of search.