how difficult is Everest base camp trek / hike to ebc & how about old people and young people

The key things to bear in mind are:

Acclimatization and Acute Mountain Sickness (AMS). A reputable guiding organisation will ensure you take a schedule that allows plenty of time to acclimatise, including acclimatization days where you camp twice in the same location but trek higher before returning to “sleep low”. They will have an expedition doctor, who will have a good understanding of the symptoms of AMS and will force you to head down if you start showing the symptoms
Heat, dust, cold and shit. The trek itself is nearly two weeks long, and throughout that you’ll be exposed to wide variations in temperature (it can be hot during the day but it’s invariably cold at night), and an environment that’s dusty and where you’re almost certain to pick up an upset stomach at some point. Coping with that over an extended period of time is almost as hard
Altitude and appetite. Stuff doesn’t taste as nice at altitude. Your taste buds are dulled. And the food that you’re going to eat is going to be carried in from somewhere on the back of a yak or a sherpa, so variety is at a premium. Forcing yourself to eat a rubbery omelette because you know you need the energy is almost the most gruelling part of the trip. Take something with a strong flavor to liven things up.
Water. Acclimatization is basically the process of your body excreting buffering chemicals from your bloodstream so your blood doesn’t get too alkaline. It can only do this through your urine. You’ll need to drink 3l or more of water each day to acclimatize properly; I was drinking 5, but then I’m huge. A good, insulated water reservoir system is a great idea (though on cold days make sure you blow the water back into the reservoir so it doesn’t freeze in the tube), and I found that (above 5,000m) hanging it on my chest inside my jacket kept it from getting too cold
The right kit goes a long way to make life more comfortable. Good boots and socks, multiple layers (including merino baselayers), a down jacket, a good breathable waterproof, a range of hats for sunny and cold conditions, very warm gloves (down mittens if you can get them), a decent down sleeping bag and silk liner, a self-inflating sleeping mat, good sunscreen and lots of alcohol hand gel will go a long way towards making the trip easier to endure.
Trekking. Beyond kit, the secret of successful trekking is simple: don’t stop. The temptation when walking at altitude is always to stop when you get out of breath. Don’t. You’ll waste energy stopping, starting and warming up after you stop, and it will take longer. Find a pace – however slow – that you can maintain indefinitely, and aim to walk for about half an hour between stops. We got the weakest walker in our group to adopt this approach, and on one of the longest ascents she went from the back of the group to very near the front.
Kendall Mint Cake. The preferred rocket fuel of the original Everest expeditions, and who are we to disagree? It’s basically mint-flavoured sugar, but on a tough climb it doesn’t take much to give you the extra boost of energy to get up it. It’s especially important on the day you go up Kala Pattar, when you start early and without a proper breakfast to get to the highest point of the trek before dawn.
Knowing what to expect. You will lose your appetite. You will get the squits. You will get dirty and cold and if you have dry skin your fingers will split open [1]. You will have headaches (though they should go away with a painkiller. If they don’t, descend immediately), and you will feel nauseous. You will have times when you don’t think you can go on. But you can get through them. When I went up Aconcagua a few years later, I found the altitude much easier to deal with – but comparing my notes from both treks made me realise that the symptoms I had in the Andes were objectively worse: I was just better prepared to deal with them.