It's not really "metabolism" as such. It's what they call NEAT, non-exercise activity thermogenesis. That's a fancy way of saying "the energy you burn day-to-day from fidgeting, walking, sitting up straight" and so on. That makes a big difference, this is why fidgety guys are runty, and laid-back guys heavier. This tends to decline as we get older. We slow down in those everyday things. If we keep up the same food intake, we put on weight.
Aside from that, recovery tends to be slower as we get older. So while the big-eating 19 year old can easily jump into 5 day a week 2 hour sessions, the 45 year old should start lower at maybe 2-3 sessions of 30-45 minutes. However, if their nutrition and rest are right, they may be able to build up to the same workload as the young bloke. Remember from other threads, nutrition and rest are most of recovery.
Older people tend to start from a lower strength and fitness than younger people. This isn't age as such, it's just they've had longer to be sedentary. Take someone who's active till 20, someone else who's active till 30, now give them ten years off - they'll be starting from a similar place. The typical 45 year old has been sedentary for more than two decades. That's a lot of time for muscle and bone to atrophy.
Age doesn't change a lot what we're capable of in the end, it just changes where we're starting from and how quickly we can get there.
The old-time strongmen used to offer routines where people would do 6-10 exercises, and start with just one set of each - adding one set a month until 3. You could do something similar. For example,
Weeks 1-4, 2 workouts, 3 exercises @ 2 sets each (1 warmup + 1 work)
Weeks 5-8, 3 workouts, 3 ex @ 2 sets
Weeks 9-12, 4 workouts, 3 ex @ 2 sets
Weeks 13-16, 4 workouts, 3 ex @ 3 sets
and so on.
Whatever combination of sessions, sets and reps you like, gradually increase the workload over time, adjusting your diet and rest to accommodate. You'll find your personal level.