The American Dietetic Association and the British Dietetic Association, the largest bodies of nutrition and diet professionals in both countries--tens of thousands of professionals--state that

a plant-based (vegan) diet is nutritionally adequate and safe for all stages of life.

Academy of Nutrition and Dietetics

It is the position of the Academy of Nutrition and Dietetics thatappropriately planned vegetarian, including vegan, diets arehealthful, nutritionally adequate, and may provide health benefits forthe prevention and treatment of certain diseases. These diets areappropriate for all stages of the life cycle, including pregnancy,lactation, infancy, childhood, adolescence, older adulthood, and forathletes. [1]

The United States Department of Agriculture

Vegetarian diets (see context) can meet all the recommendations fornutrients. The key is to consume a variety of foods and the rightamount of foods to meet your calorie needs. Follow the food grouprecommendations for your age, sex, and activity level to get the rightamount of food and the variety of foods needed for nutrient adequacy.Nutrients that vegetarians may need to focus on include protein, iron,calcium, zinc, and vitamin B12. [2]

Dietitians of Canada

A well planned vegan diet can meet all of these needs. It is safe andhealthy for pregnant and breastfeeding women, babies, children, teensand seniors. [3]

The Heart and Stroke Foundation of Canada

Vegetarian diets (see context) can provide all the nutrients you needat any age, as well as some additional health benefits. [4]

The British National Health Service

With good planning and an understanding of what makes up a healthy,balanced vegan diet, you can get all the nutrients your body needs.[5]

The British Nutrition Foundation

A well-planned, balanced vegetarian or vegan diet can be nutritionallyadequate […] Studies of UK vegetarian and vegan children have revealedthat their growth and development are within the normal range. [6]

The Dietitians Association of Australia

Despite these restrictions, with good planning it is still possible toobtain all the nutrients required for good health on a vegan diet. [7]

National Health and Medical Research Council

Alternatives to animal foods include nuts, seeds, legumes, beans andtofu. These foods increase dietary variety and can provide a valuable,affordable source of protein and other nutrients found in meats. Thesefoods are also particularly important for those who follow vegetarianor vegan dietary patterns. Those following a vegetarian diet can stillmeet nutrient requirements if energy needs are met and the appropriatenumber and variety of serves from the Five Food Groups are eatenthroughout the day. For those eating a vegan diet, supplementation ofB12 is recommended. [8]

Mayo Clinic

A well-planned vegetarian diet can meet the needs of people of allages. The key is to be aware of your nutritional needs so that youplan a diet that meets them. [9]

Harvard Medical School

Traditionally, research into vegetarianism focused mainly on potentialnutritional deficiencies, but in recent years, the pendulum has swungthe other way, and studies are confirming the health benefits ofmeat-free eating. Nowadays, plant-based eating is recognized as notonly nutritionally sufficient but also as a way to reduce the risk formany chronic illnesses. [10]

Sources are listed at CalculatedVegan.wordpress.com/authority-argot

Lastly, there is a difference between dietary risks and dietary hazards: Wageningen University - Difference between hazard and risk (video)