What instrument should I start on?

It's totally up to you!  The piano is probably regarded a 'first' instrument but you can easily start from the beginning on any instrument!

What's the youngest age to start?

I would say 6 years old. I've found in my experience that children under the age of 6 struggle to concentrate during lessons and don't reap the full benefits of lessons.

Do I have to do exams?

No, but there are definite benefits to doing so. It's the easiest way to progress in difficulty and learning scales and music theory is beneficial no matter what you want to do!

How long are lessons?

I've found 30 minutes long enough for most with only those working on the later grade exams needing a bit extra to fit everything in. As long as plenty of practise is done throughout the week, we'll easily fit new material in each lesson!

How long should I practise each day?

To begin with, I would say 10 minutes a day is sufficient. As you have more to play that should increase as time goes by. 30 minutes every day should see significant improvements each week. It's the daily practise that makes the difference, rather than the total time spent in a week!