When you're meeting people in Spanish speaking countries you probably get asked quite a bit about how old you are. This is a good question to have a snappy response to. 

Here's what I usually say…

Her: How old are you?
Her: ¿Cuántos años tienes?

You: Don't worry every part of my body still works… EVERY PART.
You: No te preocupes, cada parte de mi cuerpo funciona todavia…  CADA PARTE.

You can use this no matter what your age is.

Tips:

This is funnier if you say it with a serious face. Misinterpretation is great (especially using another language). Use it as much as possible.

Put emphasis on the last CADA PARTE. Besides putting emphasis on it I usually put my hand on her shoulder and make hard eye contact. Unless they are totally brain dead they will get it.

This type of indirect innuendo works well.

I think it's funnier if a guy says it but girls could probably use this as well.

Similar to my last post here's some positive aspects to this:

1. You've injected some sexual innuendo into the conversation without being direct.

2. You make it look like she's the one who brought up the topic of sex.

3. You've mixed some humour into the conversation and made her laugh.

4. You've avoided telling her your real age which keeps a small air of mystery going.

5. This can be used as a seed for jokes related to this later on in the interaction.