being a native speaker of english, i think my brain is permanently destroyed from memorizing noun genders (for those languages that have them). i've known german for 9 years now, and if i say so myself, am a good foreign speaker of it. i can read the paper, relate to german issues, understand german rap, blablabla... but i can never ever remember if a noun is male, female, or neuter. there are those common nouns that i've heard so many million times that i know out of instinct. for the huge majority though, i will look them up in a dictionary, or get it from grammatical context, and then forget it again. i think i subcounciously filter it out as useless information, like construction noise or other people's conversations.
I had the same problem learning French in school. I don't speak french very well however. I took my mandatory French class up until grade 9 (which was a long time ago), but never used it in day to day life, so it didn't really stick. I never remember if something is "le" or "la"
The problem is that some languages have 2 genders, but others, like Latin or German, have three.
And also beware: genders can be different in different languages! For example: in Spanish "the milk" is "la leche" (femenine), but in French it's "le lait" and in Italian it's "il latte" (both masculine).
The problem is less acute with Latin or Slavic languages, where words usually have gender-typical endings (though even there you may be tricked). Hungarian being a non-gender language, I also had trouble memorising the gender of various words in German, which seemed completely arbitrary, though nowadays I can make the right guess in about 80 per cent of cases. It would be interesting to know how long it takes for a native German child to learn the gender of all those words... A curious thing I realised, though, that German always keeps the grammatical genders of Latin loanwords, so knowing a little Latin may be a bit of help there, but unfortunately not much for native words...
Knowing grammatical gender in Spanish is pretty easy as long as you remember these rules:
*words that end in "a" are usually feminine (except for "oma" and "ama" endings)
*words that end "íon" are usually (always?) feminine
*English loan words are usually masculine (except for "la Internet")
*words ending with "o" are usually (always?) masculine
*words that end with "e" can be either (this really isn't that helpful...)
*words ending with "dad" are usually (always?) feminine
That should have been "ión".
Children acquiring a language with grammatical gender have no more difficulty acquiring this feature than any other. You just know it, along with the rest of the grammar.
Speakers of languages that have grammatical gender, however, are no more advantaged in learning another language with this feature, as learning gender in a second language always entails memorization. Having grown up bilingual, with one of my languages being a language with gender, I made quite a lousy student of German when it came to gender. I am still quite horrible at it. On the other hand, learning Spanish was easier for me, as it's more transparent.
The same goes for tones. If you speak a tonal language, it doesn't make your life any easier while attempting to learn another tonal language, I believe.
As a matter of fact, I also grew up speaking two languages, Hungarian and Serbo-Croatian, and the latter one has genders, but they are usually marked (there are gender-specific endings, albeit with a very few exceptions), and it was easier for me to learn than German, even though there is an elaborate case system there. So I agree with Vet that the problem with German genders is not a matter of whether your own language has or doesn't have genders, but the fact that the words are usually not marked for gender, although there are some exceptions here, too. German genders depend more on word category (e.g. the names of days are and seasons are masculine, although Woche, 'week', is feminine). And, curiously, Mädchen, 'girl', is neuter, but only because '-chen' is a diminutive suffix, and all diminutives are neuter as a rule. The original root word, 'Mädel', is feminine, and it is a cognate of English 'maiden'.
Sorry, I was wrong with this last one, I meant the word 'Magd', cognate of English 'maid', which can mean both an unmarried girl and a maid. It is feminine. On the other hand, 'Mädel', which means 'young girl', is neuter (!).
We have 3 genders, but they are quite logical, at least to a native speaker. Foreigners would probably have problems with them.
About the gender rules in Spanish:
Things are more complicated than you can see there. I'd put the rules like this:
* Words ending in "a" are usually femenine, except "el día" (the day), and Greek loan words, like "sarcoma" or "problema".
* Words ending in "o" are generally masculine, except "la mano" (the hand), "la dínamo" (the dynamo), and other words.
* Words ending in "n" are generally masculine, but if they end in "ción" or "sión", they are femenine.
* Words ending in "dad", "tad" or "tud" are femenine.
* Words ending in "e" are masculine or femenine. You have to learn the gender!!!