Remember that your starting item is spell thief's your primary job is to harass the enemy and the more you do the more gold you get. Rod of ages goes well with your passive that scales on health. If you play well and land skill shots well, you'll be able to get Rod of ages within 12-14 minutes. Liandry is good but remember you're a support, your adc finishes the enemy laner, Liandry is useful for roaming and teamfights which you'll rarely get during laning. You need the health and Mana for your early game that Rod of Ages gives