sodapple wrote:I would like to know if Japaneses really know all kanjis
Answer is NO
they learn and forget,
most japanese people I know have kanji dictionaries in home or electronic dictionaries.
it is the same as english people have english dictionaries. you can try online dictionary.
http://dictionary.reference.com/kanji have many readings, japanese people may not know all of them.
example if kanji character has 3 readings they may know only one or two of them.
to know how kanji looks like and stroke order to write it does not mean they know everything about kanji.
about the number of kanji
from my research I assume that japanese people know about 600-800 kanji well. as for the rest they may have incomplete knowledge about them. of course it depends on how much time they spend learning kanji. some people may really know about 2000 kanji or more but I doubt they have the time to learn more useful things. Kanji learning takes too much time of which they could spend on learning more practical things.
If you want to ask a Japanese person about the number of kanji they know ask yourself how many english words you know first. probably you can't answer.
some people mentioned kanji used in japanese names.
the truth is that they may use non standard kanji outside of the kanji list including old type Kyuujitai. the reading may be different too. japanese people often ask how to write persons name.
example: Sakurai can be written 桜井 or 櫻井
from a daily life, in some restaurants when there is a queue people are asked to write their names and they write them using kana. I don't know exactly what it is about maybe they book a seat. it is not the point here.
There are many more examples. names may be made out of one, two, three or more kanji.
sodapple wrote:in what age they finish to learn all kanjis
they never stop to learn kanji.
just think about it, there is no possibility for any person to remember them all.
If you want to count the chinese characters there will be about 3k and if you include multiple versions of transformed, simplified, artistic and archaic characters not used anymore then that number could reach 100k or more than that. The Taiwanese dictionary contains over 106k characters.
Simplified chinese - about 2300 + 1000 obsolete but used in some regions
discovered from about 6000 BC - 3100 characters
the rough number of characters in the set is 3000.
there are many countries that use their own different sets of characters like
China (Mandarin, Cantonese, Wu[Shanghainese]), Japan (Kyuujitai, Shinjitai), Singapore, Taiwan, Hong Kong, Malaysia ...
the list goes on