Are there different prices for seat reservations depending on age?

The price for seat reservations is always the same, regardless of age.

This is why you might sometimes see an error message like, "You have been assigned a different traveler type than the one you selected due to the specified age (e.g., Adult instead of Child/Youth). Please check your booking." when booking seat reservations.

This is nothing to worry about, and you can ignore the message and proceed with your booking.