How to respond to compliments about my Japanese in informal way?

I learned Japanese on my own and reached a good conversational level. One thing I learned though is that if you were complimented about how good you are in Japanese by natives, you respond: いいえ、まだまだ！

That somehow worked for people older than me, but among people I meet who are my age or younger, they seems to giggle or laugh when I say this. It’s like I’m answering so seriously!

I tried to to accept the compliment by nodding and saying どうも！But still I fear it’s forward (since we are talking about Japanese people here).

I don’t know which is the best way to respond to compliments or questions like “why/how you are that good in Japanese?” that is not considered weird.