$\textbf{BACKGROUND}$: older people may be less likely to receive interventions than younger people. Age bias in national guidance may influence entire public health and health care systems. We examined how English National Institute for Health & Care Excellence (NICE) guidance and guidelines consider age. $\textbf{METHODS}$: we undertook a documentary analysis of NICE public health ($\textit{n}$ = 33) and clinical ($\textit{n}$ = 114) guidelines and technology appraisals ($\textit{n}$ = 212). We systematically searched for age-related terms, and conducted thematic analysis of the paragraphs in which these occurred ('age-extracts'). Quantitative analysis explored frequency of age-extracts between and within document types. Illustrative quotes were used to elaborate and explain quantitative findings. $\textbf{RESULTS}$: 2,314 age-extracts were identified within three themes: age documented as an $\textit{a-priori}$ consideration at scope-setting (518 age-extracts, 22.4%); documentation of differential effectiveness, cost-effectiveness or other outcomes by age (937 age-extracts, 40.5%); and documentation of age-specific recommendations (859 age-extracts, 37.1%). Public health guidelines considered age most comprehensively. There were clear examples of older-age being considered in both evidence searching and in making recommendations, suggesting that this can be achieved within current processes. $\textbf{CONCLUSIONS}$: we found inconsistencies in how age is considered in NICE guidance and guidelines. More effort may be required to ensure age is consistently considered. Future NICE committees should search for and document evidence of age-related differences in receipt of interventions. Where evidence relating to effectiveness and cost-effectiveness in older populations is available, more explicit age-related recommendations should be made. Where there is a lack of evidence, it should be stated what new research is needed. ; Other ; This work was supported by the National Institute for Health Research's School for Public Health Research (NIHR SPHR http://sphr.nihr.ac.uk/). J.A. & M.W. are members of the Centre for Diet and Activity Research (CEDAR) a UKCRC Public Health Research Centre of Excellence.