My take on it is that the best age to do the sealing is 4~5. Any younger and there is not enough sense of self to resist mental pollution even if an Uzumaki, at least when it's from a full-power Kyuubi; any older, and a Bijuu of the level of the Kyuubi will not integrate with the Chakra network well enough (or, rather, the Chakra network can't adapt well enough for the Jinchuuriki to be able to use its power to the fullest). Thus, Naruto was too young to be able to mentally resist, so Minato took measures.
The 'only at certain ages' and 'only Uzumaki' in my headcanon gets more lax the weaker a Bijuu gets, and even more lax with common points between Bijuu and Jinchuuriki-to-be (a potential host of the Two-tails having a fire affinity, for example). I'm well aware that Magoose has different headcanons, but I still want to get my opinions out there.