Question
QUESTION: I just seeded my lawn Easter weekend and now it has started to come up. Problem is that existing grass and weeds have come up gangbusters also. When should I mow or can I mow at the highest setting on the lawn mower. Lawn looks like a jungle after all the rain we've had

ANSWER:  An accepted standard is that not more than 2/3 of the grass height should be removed. This means that if your mower's highest setting is 4" then the grass should not be more than 6" when you mow. From how you describe it, the lawn appears to need a mowing now.

---------- FOLLOW-UP ----------

QUESTION: Also when can I fertilize again. Last time I fertilized was first week of February with Sta Green Winterizer which I found leftover from the fall. I was going to fertilize now  with Scotts Turf Builder with Weed Control. I did not put down starter fert. when I seeded. Should I hold off or go ahead

Answer
The first concern is to get the grass firmly rooted in. I imagine that you are watering regularly and will continue to do so for another month or so (or until rainfall is fairly regular}. Therefore you should apply something organic and not very soluble. "Milorganite" is both.