いくつになったの？って英語でなんて言うの？

Yes, the topic of discussing someone's age may be a delicate one depending on the person involved. It is probably a decision you will have to make yourself. However, in general, it is considered impolite in the UK to ask a mature woman her age - unless that is a specific topic in a conversation, or unless you know this person quite well. Men are usually less sensitive about their age and more willing to discuss it, but it is a personal detail and may be taken as an intrusion into personal privacy.
"Happy birthday! May I ask what age you are celebrating?"
"I never discuss my age, sorry."

>Happy birthday, how old are you now if I may ask?
*This is asking in a polite way how old he/she is now.

>Happy birthday, What is the lucky number?
*This is an informal way of asking. "Lucky number" is referring to the "new age".

Instead of saying"Anyway how old are you going to be?" you can say "Anyway how old are you now?"
You can use the following sentences to express yourself:
Happy birthday. How old are you now?
If you don't mind me asking how old are you now?

Both polite questions as it is a very awkward question to ask. More informal almost slang with a friend could be something as simple as how many candles are you burning(birthday cakes have candles).

An amusing way to ask "how old some one might be"...(as used for cars)
How many "miles are on the clock!" some people " hate to discuss their age".
and it can be sensitive subject as we get older...Especially among ladies!