If staying on the latest model isn’t important to you, there’s significant cost savings to be made from just buying the previous version.
So owning a device for 6 years between age 1 and 7 will generally have a lower cost than owning a device between age 0 and 6.
For Apple products it’s generally feasible to effectively buy first hand devices aged 1+ because they’re still available for sale (at least in some retailers) after a new edition is released.