OP, I don't think the ages of your child has much to do with fulfillment unless you have a litter of under fives, in which case you're just trying to survive and don't have time to ponder philosophical issues such as happiness.
Not sure if you're a SAHM or not, I'm on mattie leave just now and loving it (hoping for lottery win) but when I'm back at work I have a pretty good life balance, I work 5 mornings a week and have afternoons at home. I loved it when DS1 was 2, they're growing so much, learning to speak and jump and it felt like every week he changed.
I don't think you can expect motherhood by itself to fulfill you though. We're financially okay, my DH is incredibly hands-on and a great support to me, we have a great relationship which has only gotten closer through having the kids (whereas I read on here a lot that some couples drift apart when kids are on the scene), and I am happy with my family life (parents/siblings etc) and what little social life I have :-)
All these things probably make it easier for me to enjoy my kids, maybe there are other aspects of your life that have changed since you had your child and that you're not happy about, maybe if those changed you'd enjoy 'motherhood' itself more?