Comprehensive Flutter App Development

I'm seeking an experienced Flutter developer who can create an application suitable for an array of platforms; Android, iOS, Web, Windows, and Mac. The application will need to be versatile, with functionality that appeals to several demographic groups - teenagers, young adults, the elderly, and even kids. The ability to understand and cater to these different user interfaces will be crucial.

The app must entail:
- User Authentication: Freelancer must be adept at creating a secure, robust login module.
- Push Notifications: Freelancer should have expertise in implementing real-time push notifications to enhance user engagement.
- In-app Purchases: Past experience in setting up in-app purchasing systems will be needed.
- Referal Code System: Freelancer should be able to set up a referal code system to encourage users to invite others.
- Image and Video Upload Capability: Freelancer must be capable of integrating media upload features within the app.

Skills and experience in Dart and knowledge on Flutter's framework will be beneficial. In addition, understanding user interface design for different age demographics, and in-depth knowledge of multi-platform app development are desired. Potential freelancers should be able to present a portfolio of similar projects as part of their submission.