There is no 'right' age for children to start gardening. When they are toddlers, they will naturally want to be with you outside and will join in with your activities – digging, helping to push the wheelbarrow, watering plants, adding weeds to the compost, and so on.
Growing seeds like sunflowers or cress on the windowsill are fun, or planting sweet peas – or growing and watering herbs is a lovely activity for little ones.
As children garden, they develop important fine and gross motor skills that will help them improve their academic skills such as writing, cutting and dexterity. It's a well-known fact that if you engage children in growing their own vegetables, they gain a keen interest in eating them too!
Going to the shops to pick out the herbs, plants, flowers or seeds (which also involved pushing a mini trolley around the garden centre can be very exciting for toddlers.
Start small and enjoy the joy of watching something spring into life.
Also some lovely books to read together are 'The Tiny Seed' by Eric Carle
Roots Shoots Buckets & Boots: Gardening Together with Children by Sharon Lovejoy