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Most people get more than enough magnesium from foods and do not need to take magnesium supplements. Excessive use of magnesium supplements can be toxic. In addition to what you get from food, the highest dose you should take of magnesium supplements is:

65 mg/day for children ages 1-3
110 mg/day for children ages 4-8
350 mg/day for adults and children ages 9 and up
These doses are the highest anyone should add to their diet. Many people ingest significant quantities of magnesium through the foods they eat. It’s safe to get high levels of magnesium naturally from food, but adding large amounts of supplements to your diet can prove dangerous. Do not exceed these maximum advised levels.

Wrong. See post #19. There are chronic deficiencies the world over, including in the US.

From second study posted:

Importantly, much of the population may not even be meeting the RDA for magnesium. For example, the average magnesium intake in men and women in Taiwan is only 250 mg and 216 mg, respectively, or 68%–70% of the recommended dietary reference intake. The authors of the study concluded: ‘Taiwanese elderly persons had suboptimal levels of dietary magnesium intake, which although may be sufficient to avoid overt magnesium deficiency, may not be sufficient to reduce the risk of diabetes in the elderly’. The National Nutrition Survey in Japan found that for people 15–49 years old the intake of magnesium was below the Japanese RDA. Moreover, around half (48%) of the US population consumes less than the recommended amount of magnesium from food. The US Department of Agriculture states that the average magnesium intake in women and men is 228 mg/day and 323 mg/day, respectively. Based on these data, one group of authors noted: ‘These intake levels suggest that a substantial number of people may be at risk for Mg deficiency, especially if concomitant disorders and/or medications place the individual at further risk for Mg depletion’. The most recent published review on magnesium concluded: ‘Approximately 50% of Americans consume less than the Estimated Average Requirement (EAR) for magnesium, and some age groups consume substantially less’.

Lakshmanan et al collected daily food records for 1 year in 34 men and women. They found that the mean magnesium intake was 323 mg/day and 234 mg/day in men and women, respectively (around 4 mg/kg/day). Despite this level of intake, the overall patient population was in negative magnesium balance (−32 and −25 mg/day, respectively). Moreover, 75% of women consumed less than the RDA (300 mg/day) and only one out of the eighteen women were in equilibrium. Considering that the average intake of magnesium in the USA is around 228 mg/day in women and 266 mg/day in men,a large percentage of Americans may be at risk of negative magnesium balance. Indeed, ‘Only American diets containing more than 3000 kcal/day may provide 300 mg or more magnesium’. Another long-term study lasting 50 weeks found that somewhere between 180 mg and 320 mg of magnesium/day is required in order to maintain positive magnesium balance. And since many individuals may be consuming below 320 mg/day of magnesium, this poses a major public health threat.

The data are consistent around the world that magnesium intake may be inadequate. Indeed, the intake of magnesium in Germany was determined to be only 200 mg for women and 250 mg for men. In Kiev a study on 780 men aged 20–59 found that the overall population consumed insufficient magnesium from food rations (10% less than the recommended value). The authors also found a correlation between the low magnesium consumption in food and the prevalence of risk factors for ischaemic heart disease, such as hyperlipoproteinaemia, arterial hypertension and body weight. In 2004, Durlach concluded: ‘About 20% of the population consumes less than two-thirds of the RDA for Mg. Women, particularly, have low intakes. For example, in France, 23% of women and 18% of men have inadequate intakes. Mg deficiency during pregnancy can induce maternal, fetal, and pediatric consequences that might last throughout life’. The median magnesium intake in an elderly population (832 subjects aged 70 years or older) in Southern France was also found to be below the RDA. Another French study on 2373 subjects (4–82 years of age) noted that 71.7% of men and 82.5% of women had an inadequate magnesium intake.