Level of competence

You asked

if an individual who has 20 years plus of experience with both methods and hands on experience and is really knowledgeable on the subject, it that sufficient to provide responsible sign off of product even although the individual is not a doctor or a professional body member etc. 

ISO 17025 requires personnel competency, however laboratory management must define the competency requirements for this activity. You need to base this on your sector, applicable legislation, and requirements from the accreditation body. For many sectors, highly skilled an competent personnel are not necessary member of a professional body. In other sectors it is mandatory. 

What is required by ISO 17025 is that personnel must have assigned, communicated and well understood responsibilities an authorities. 

Typically a laboratory will appoint a competent “signatory” who takes responsibility for the technical validity of results, and they sign the report. Bear in mind that depending on the size of the laboratory, other personnesl, such as a quality controller may be responsible for the quality control of a particular batch and may “sign off” the batch analysis as valid as the QC passed. The laboratory’ s quality assurance process should specify this, meaning what objective checks and responsibility does a  “sign  off” versus report “signature” indicate. Often the laboratory manager will co-sign a test report, taking overall responsibility for the approval of a report to go to a customer

You also asked

Is there a document that would define the requirements to sign off on testing and results etc.

The specific technical requirement relates to technical quality assurance (control of the test method)., covered by ISO 17025 clause 7.7, Ensuring the validity of results. This should not be seen as the only requirement. It also requires knowledge and evidence that the overall Quality assurance of, for example suitable equipment, conditions, methods and metrological traceability is in place. Although these overall responsibilities may that of say a Quality Manager, the person approving the release of results should have evidence that the management system quality assurance is effective. 

The following toolkit documents could assist you

Quality Assurance Procedure at https://advisera.com/17025academy/documentation/quality-assurance-procedure/
Competence, Training and Awareness Procedure and the four related appendices, available at https://advisera.com/17025academy/documentation/quality-assurance-procedure/
Testing Report Procedure at https://advisera.com/17025academy/documentation/testing-report-procedure/