Don't ask, don't tell? (The age version)

Is your age relevant to your particular query? I've not been advised to include my age, ever. Just thinking...and welcome!

Age isn't important unless, as noted above, it's relevant to your story.

just include those activities in a short bio should be enough. They are impressive even without the age.

Don't tell your age. Your work needs to be impressive regardless of whether you're 19 or 91. It could be a nice marketing gimmick later on, but an agent or an editor will not be overly impressed of your age. They're more likely to read it that you're asking to be viewed with a special lens - many young writers write really well for their age, but compared to all the other books on the shelf they still have a lot to learn. This is not to say that applies to you, but you need to consider that an agent/editor is going to immediately assume you are asking for special consideration. Given the types of books they typically see from young authors, they may approach yours already with the bias that it will be no good.

Just let your book speak for itself. If they like it, and you feel like boasting about how young you are, or you think it might be useful for marketing purposes, I'd say do it after they start working with you. Before they've read your stuff, young age is probably more likely to bias them against you than it is to excite them.

No relevance. You're not underage so you don't even need a guardian. Even so, the question is irrelevant until you need to sign a contract...

I wouldn't.

And I also suggest that you post your query letter in the workshop area of this website, where you'll receive plenty of advice which would promptly increase your chances of success! Good luck.

I wouldn't mention it unless you are writing YA or for kids. Otherwise, agents really don't care.

Oh, and of course!

Age isn't relevant until they send you a contract to sign and you're under 18. Then you'll need your parents to counter-sign it.