In a healthy, full term infant, iron stores, together with what's in the milk (commercial formula or breastmilk) is ample to meet baby's iron needs for most of the first year.
Solids are usually started around 6 months because it gives baby a chance to practice and become adept BEFORE they really need them, and because most babies are amenable/able to eat them at that point, and because most baby's digestive systems can now handle non milk foods.
(Worries about iron date back to when most babies were fed home-made formulas. These formulas contained no iron [they were just regular cows milk, sugar and water], so babies did tend to become anemic. But breastmilk contains small amounts of well-absorbed iron, and commercial formula contains large amounts of poorly absorbed iron. So for most babies, that's plenty. If baby is eating no solids (or only iron-poor solids) by 9 months, it may be prudent to get her iron levels checked, but most babies will still be ok.
If you want to wait longer, or baby isn't interested, that's fine too. The AAP in the U.S. specifically states that some babies wont' want them until 8 months.
Once solids ARE started, they are mostly just for fun, and they don't replace breastmilk or formula. So the WHO recommends breatsmilk alone for the first 6 months, add complementary foods (solids) after 6 months, and continue to breastfeed for a minimum of a year.