You can apply for OINP now with your existing NOI. It is valid through December 31. Don't worry that you are under 400 points now. OINP locks your age as of the date of your NOI, so you are still eligible as long as you are still eligible for EE. See page 31 (page 36 of the PDF) of the application guide:
http://www.ontarioimmigration.ca/prodconsum/groups/csc/@oipp/documents/document/oi_en_hcps_010517.pdf
4. Age
A maximum of 12 points may be awarded based on your age on the day when your application for permanent residence is received by IRCC.
Note: The OINP locks in your age on the date that your Notification of Interest (NOI) from Ontario was sent. Therefore, the OINP will award points based on your age at the time of your NOI. However, please note that Immigration, Refugees and Citizenship Canada will only lock in your age once you receive an Invitation to Apply.
Yes, after the age of 40, we lose 11 points per birthday if single and 10 if married. Btw it's nice to meet another 40+ person. There aren't many of us around here. My birthday in February will bring me down to 399, so I am really praying that I will receive nomination (and hopefully ITA) before then. Once we are below 400, only another ICT exception week like what they did in the last week of June can fetch us another NOI.