The thing is, I think it's funny. You cannot say whether something is funny or not, only whether it's funny or not to you.
LOL! ROFL! – He who laughs last, laughs longest. Your thesis has proven right again: What is funny depends on the subjectivity of the respective individual.
Well, I for one think that´s a good one, nay one of the best ever! It has even got a tinge of the tragicomic. You will certainly not find it funny. But that´s really unequalled: a harmless question has changed your identity. Meantime, you have mutated into an English native speaker. That´s what science calls evolution! (A former thread member will be in raptures about the phrase evolving language and feel confirmed in her views.) Linguists, who will rejoice, should be made aware of this rare phenomenon of a speaker´s transition from one native language to another, in doing so even skipping the borderlines of different linguistic branches, from Slavonic to Germanic. In face of the unfortunate outcome, however, they will lament the progress achieved by speaking two mother tongues (just two?). Here is a real-life example (quotation):
Location: Toronto, Ontario (Canada)
Native language: Canadian English, Russian
If you still know him now, you would say:
I knew Tom since when he was just five years old.
I have known Tom since when he was just five years old. ( I would rather use this.)
This lady doesn´t speak English, but volumes about her "native language"!