I don't think age is the determining factor.
But, I guess you are an adult from today.

A new you.

I am sure that from now on, more than ever
I am sure that you will have more fun, and less fun, than ever before.
I am sure that you will experience a lot of fun and not-so-fun.

I hope you will be happy.

I always hope so.
I want you to be happy.
I give you a handkerchief.

Wipe your sweat or wipe your tears.

As you use it in this way
it becomes more and more comfortable in your hand.
I think it will gradually become more comfortable in your hand, and will change to a softer look.

Failure is part of success.

No one is perfect from the start.
We have to move our hearts and minds a lot.
and walk through each day.

Congratulations on your coming of age.