Honestly, as soon as you're both ready. If you've been an avid hiker since before children, it should be an easy transition from baby hiking to toddler hiking. There's no fixed age, and no matter if they are highly independent walking alone or prefer to be carried at any possible moment, start with short hikes and work out what is comfortable for you. Don't be afraid of regression, either. If they were bound full of energy last month, yet now they refuse to move, don't give up! Maybe it's the weather, time of day, or lack of company. Try another of the strategies I mentioned above and if it's not working and little legs refuse to budge, have a backup plan and try another day.