what is the age limit for it and how long do we stay for or do we choose.and general cost for scuba diving with full equipment

You can’t go on a dive trip or rent/buy equipment without being certified. So I would suggest you get certified before your trip, or get certified while you’re in Malta. Also, some places will take non-certified people on a "discover scuba" trip, but I don’t know much about that. You might also consider "snuba" which involves an air hose that goes up to the boat, instead of a tank on your back – I’ve never done that, either, but I don’t think certification is required for it.

There are several certifying bodies. I got mine through PADI (www.padi.com) and they require you to be at least 10 years old, but if you’re 10-14 you’ll get a junior certification (which you can upgrade to an adult certification after turning 15). I think the certification cost me about $350 including equipment rental, admission to the place we did the diving, etc.

Costs vary. In Costa Rica I think we paid about $60 per dive for the dive trip, and close to that much for equipment rental. It’s also customary to tip your divemaster. We’re going to Hawaii this winter, and it’s more like $120 per dive.

Most people do 2 dives per day. Leave about 8am, back by noon. You can do that for as many days as you like. But if you’re talking about doing it for a week, then maybe look into a live-aboard boat trip instead, and you can probably get more dives in, or go to more interesting places.

Note: My costs are in US dollars.