A: Yes to both questions.
If you want to force the entry of a field, for example the Date of Birth you can do so by setting the 'Validate Customer Details when hire vehicle out, convert quote to booking' system parameter flag to Yes. This will ensure that the system will check that all relevant customer detail is entered during the booking process.
Insurance excesses can be set up to be aged-based, i.e. hiring a vehicle to a U25 will automatically show available excesses for that age group.