Which would you choose and why?
Option A : £50k in cash at 18.
Option B : £150k in cash at 25.
Option C : £1mil in cash at 30.
Option D : £200 per month at 16, £1k cash + £300pm at 18, £450pm + £70k at 25 and £900pm + £250k cash at 30, the £900pm will then go up every year at CPI and every 5 years CPI+5%
If you select option D then you can't change it but it doesn't prevent you from taking up any steps to gain more in income.
If you select any other then for as long as you had not taken the money you could opt not to take it and wait till the next option.