I would have thought 18 weeks is too early. I understand it that they should move gredually on to adult food when they are 1 year old.
As they are still growing I think they still need the nutrients found in kitten food. Are they outside cats? The will definitely need food that gives them energy if they are.
I think what you are feeding them at the mo is fine. Mine are 8 months, have been neutered and are indoor cats. They are not fat at all but just filled out as they should for their age.

KK

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Karen
Slave to Tommy & Jessy born 13th Feb 2009