It's common for most people coming from sea level to experience at least mild symptoms of altitude sickness when they arrive in Cusco, which is 11,000 ft/3430 m above sea level.  Age is not a factor for the severity of symptoms one can experience.  But fitness can be. Symptoms vary from person to person, but can include:

• headache
• nausea
• insomnia
• lethargy
• loss of appetite. 

Treatments may include:

• OTC pain reliever - bring your own favorite remedy for pain relief
• Oxygen. Most hotels have oxygen tanks in their lobby. Some local pharmacies sell an individual size O2 tank for personal use.
• Coca Leaf. Locals believe that drinking coca leaf tea will help and you will find the tea leaves readily available in hotel tea bars as well as restaurants. 
• Water.  Drink extra to keep your blood flowing freely.  Dehydration leads to sludgy blood and may contribute to your discomfort.
• Diamox. (AKA "Acetazolamide") can be purchased by prescription in some western countries or over the counter in Peruvian pharmacies. 
• Treatments may alleviate the symptoms as you adjust in the first 24-48 hours.  But you cannot expect the feeling of breathlessness to completely disappear, no matter how you treat it. In rare cases, individuals may need to seek medical attention. Clinics are located in Cusco.

Is it safe to drink the water?
Tap water in Lima, Cusco and other larger cities is consumed by local people every day.  But is it safe for you?  There's only one way to find out and we suggest that you not drink the tap water or use it to brush your teeth.  Bottled water is cheap and available everywhere including small towns and villages.  We support Travelers Against Plastic to reduce the use of disposable water bottles and encourage you to bring a purification device to make your own safe drinking water.  The device should be rated to purify water from microbes and viruses, not just for taste.  Purified drinking water is provided on all the multi-day hikes and during your stay in the Amazon jungle lodges.

Should I travel to South America if I have a life-threatening allergy?
We've learned that there are different kinds of allergies; from hayfever to insect stings.  IF YOU HAVE A LIFE-THREATENING ALLERGY TO ANY FOOD, INSECT, MEDICATION OR NATURAL ELEMENT, WE REQUIRE WRITTEN, ADVANCE NOTICE WITH FULL DESCRIPTION OF THE ALLERGEN AND THE TREATMENT. Be aware that Epi-pens may only useful in areas where you are close to medical services like Lima or Cusco.

What is the best way to travel between Lima and Cusco?
We recommmend flying between the cities because other means requires more time or is not practical for short vacations.  There is no train service between Lima and Cusco.  And while there is bus service, the drive is almost 24 hours long over paved, but winding mountain roads. There is no speedy highway.  The cost of round-trip air tickets varies from around $250 pp on the domestic carriers, including STAR PERU AIRLINES and PERUVIAN AIRLINES, to $450 pp on LATAM AIRLINES and somewhere in-between on AVIANCA AIRLINES. The majority of the flights between the cities occurs in the morning early evening hours.  We don't recommend booking the last flight of the day due to the risk of delays because of mountain weather conditions.

Where should I stay if my international flight arrives in Lima in the evening?
There are no late evening flights from Lima to Cusco. And there is only one hotel at the Lima airport. The Wyndham Hotel is connected to the arrivals terminal by a skybridge on the 2nd floor.  It takes literally 2 minutes to walk from the terminal to the lobby of the hotel.  The hotel is expensive by Peruvian standards (in excess of $250 per double room per night). We've seen good rates on Booking.com and Expedia. The hotel will accommodate as many as 3 to a room, but not 4. 

This hotel is best for groups who arrive in Lima late evening and plan to depart again by air the next morning.  For groups planning to stay in Lima, it's best to pre-arrange a transfer to a hotel outside the airport, unless you don't mind paying the "gringo rate" when you negotiate on your own. The travel time is about 45 minutes each way when the traffic is moderate.  And the cost ranges from $35 each way and up.  If your group is larger than 3 persons, you may need a transfer in a mini-van or two taxis. 

Do I need vaccinations to visit Machu Picchu?
The altitude of Machu Picchu is 8000 ft/2440 m and higher.  Disease-carrying vectors do not thrive at higher elevations.  US passport holders are not required to get Yellow Fever vaccinations or prophylactic Malaria treatments.  Although there have been no recent cases of Yellow Fever in Peru, some countries, including the US, recommend that visitors get Yellow Fever vaccinations prior to visiting the Amazon Rainforest including Tambopata Reserve or Manu National Reserve.  We recommend that travelers check the regulations for re-entry for any countries they will pass through after leaving Peru, Bolivia or Ecuador.  Bolivia requires all foreigners to have proof of yellow fever vaccination upon entry if you plan to travel to the Bolivian Amazon jungle. (Some countries including Australia, require their citizens to get yellow fever vaccination if they enter Peru, even if they are only visiting the higher elevations)