Hi Jackie
My daughter (23 at the time) did it last year and summitted successfully - but she described the final ascent vividly as walking up a really steep slope for 7 hours mostly in the dark, in unimaginable freezing cold, suffering from a severe bout of 'flu!! She did have a great time though - in retrospect!!
I think one of the problems is that very few people allow enough time to acclimatise to the altitude, and if you can find a trip that climbs a nearby peak first for training, it should be better - though obviously more expensive.
Altitude sickness has little to do with overall fitness and sometimes affects young people more than it does older.
It looked and sounded amazing though, so if you do decide, have a brilliant time
Paul