Usually people are worried about age discrimination against older people, but I've certainly seen workplaces where people above a certain age fit in and younger people did not.
In fact, employers can have legitimate reason to ask your age, such as background checks.
"I would like to focus on the value I can add to the company instead of focusing on my age"
is this an acceptable answer?
So are questions about marital status; about whether you have kids or are pregnant; about race or nationality; about gender; about religion; and about disabilities.
I'm sure the interviewer had no idea. She was German, maybe it's not illegal to ask these questions in Germany
Here are some of the relevant links to your points:
https://www.eeoc.gov/eeoc/publications/age.cfm says: "The ADEA does not explicitly prohibit an employer from asking an applicant's age or date of birth. However, such inquiries may deter older workers from applying for employment or may otherwise indicate possible intent to discriminate based on age, contrary to the purposes of the ADEA."
https://www.eeoc.gov/laws/practices/inquiries_marital_status... "The following pre-employment inquiries may be regarded as evidence of intent to discriminate when asked in the pre-employment context: * Whether applicant is pregnant. * Marital status of applicant or whether applicant plans to marry. * Number and age of children or future child bearing plans. * Child care arrangements. * Employment status of spouse. * Name of spouse."
https://www.eeoc.gov/eeoc/publications/fs-race.cfm says "Requesting pre-employment information which discloses or tends to disclose an applicant's race suggests that race will be unlawfully used as a basis for hiring. Solicitation of such pre-employment information is presumed to be used as a basis for making selection decisions. Therefore, if members of minority groups are excluded from employment, the request for such pre-employment information would likely constitute evidence of discrimination."
Execuse me :)
Therefore, most HR departments simply tell you that it's illegal (or prohibited, more precisely). There's a huge amount of legal risk to the company, in asking, and no benefit because it is illegal to actually take the answer into account.
Personally, I would probably answer truthfully in the interview, and then politely tell the recruiter / hiring manager next time I spoke to them, "Thanks, I enjoyed speaking with <person> about the company! By the way, they asked me about my age (or whether I was married, or whatever), and I just wanted to confirm that this information will not be used in a hiring decision. We were making small talk and I didn't mind answering, but I don't think it's relevant to what I can bring to the company." If the recruiter or hiring manager is in the least bit competent, they will immediately panic, and probably end up finding a way for that person to have no role in making your hiring decision, or something.
(This only applies for hiring in the US, if this is in another company things may be totally different)