The age of the plant isn't important. Water ph depends on soil ph. If your soil ph is above 6.5 than 6.5 ph water is ok, if you soil ph is below 6.5 you might want to bump the water ph up. In the beginning the ph should not change much in your soil, when you start feeding the ph can change quickly. If you don't know your soil ph you will have to water the plant enough to get a fair amount of runoff, unless the leaves look bad don't water a lot. Overwatering seedlings will kill them quickly. Water 6.5 to 6.8 should be ok unless your soil ph is real high or low. hope this helps.