As with all subjobs, the max level as a subjob is half the Main Job's level. It does not affect the actual level of that class.
I.e. You get your White mage up to level 65, and start your Summoner at level 1 the day after you defeat the last of your summons to get the ability to use them as a summoner. If you sub your White Mage, you will immediately be a Level 1 / Level 1 Summoner / White mage. As you level your summoner, the "current" level of your White Mage will increase to match the maximum, half. For example, the minute you hit level 4 as a summoner your white mage subjob will be level 2, without you having to do a thing.