"How old are you?"
You can ask としはいくつですか. You write thisとし with 歳. That kanji is readとし when it's alone andさい when it's got company, like 何歳(なんさい), where it is a counter for age years. So don't try to use it alone or you may be misunderstood. さい by itself (with different kanji) means also a rhino or dice.
From what I have seen, 「おいくつですか。」 is used exclusively when conversing with small children (full stops are more formal than question marks in Japanese). According to the Japanese Stack Exchange, 「お歳をお聞きしてもいいですか。」 is the most appropriate way to ask a stranger their age, but females merit more caution because it is considered impolite to ask them their age. Check this link for more information: https://japanese.stackexchange.com/questions/2749/polite-way-to-ask-how-old-are-you-%E4%BD%95%E6%AD%B3-%E3%81%84%E3%81%8F%E3%81%A4-%E5%B9%B4%E9%BD%A2-%E3%81%94%E5%B9%B4
Ok, I just asked my Japanese husband about this! Here is his answer. いくつ is more general, directly meaning "how many?" It is also more childish. The equivalent in English is when a very young child holds up 3 fingers and says, "I'm this many!" 何さいですか is specifically asking about age while いくつ can be asking about the quantity of anything. Also, if you ask an adult, [いくつですか] they are going to look at you a little weird for using baby talk at them. 何さいですか is acceptable for all ages and is neutral on the politeness scale. おいくつですか is acceptable for adults and therefore polite, but you have to use the お。 TLDR: 何さいですか is a safer bet but おいくつですか is acceptable too.
I had a Japanese teacher who mentioned that being too formal can sometimes make you sound condescending or closed off in some situations. So I'm assuming this phrase would best be used with someone who is your superior by a LOT, and not people who are on the same level as you.
in russian it would sound like '[to you] [how many]' in the most casual form (and '[how many] [to you] years' in the polite form) literal 'how old are you' would sound incredibly rude like... you're either calling someone old or questioning whether they're an adult and just... bad form