Hi Gareth, Have you looked into this?

Per document type 

You can set decay parameters per document type using Client Conventions. 

client.Conventions
  .ForInstancesOf<BlogPost>()
  .SetDecayParameters(new DateDecayParameters(TimeSpan.FromDays(60)));

This applies the decay parameter for document of the specified type and all sub-types. When you use per-type decay parameters, the default scale parameter (UsingAutoBoost()) is optional.

Excerpt from:
https://world.episerver.com/documentation/developer-guides/search-navigation/NET-Client-API/searching/auto-boosting/

Thanks @dada,

unfortunatly that option doesn't work with Unified Search.

But I have found the solution to the second part of my question, which actually works out better for us as I can let the editors select which pages should and shouldn't decay, by overriding the SearchUpdateDate.

According to this page Unified Search | Episerver Developer Community "If you cannot determine a date value, the decay defaults to the middle of the range, by default 0.6."

From the above statement and my testing it looks to me like newer pages effectivly get a boost, older pages get decayed and pages without a date sit in the middle, which is perfect for our scenario

Cheers,

Gareth