You asked two questions. First,
I understand that lenders cannot discriminate based on age, but is there really a data to support that lenders do not do that ?
Considering your United States tag, it's worth noting that the Equal Opportunity Credit Reporting Act, the Fair Housing Act,and the Home Mortgage Disclosure Act were intended to prevent discriminatory lending practices. As part of those regulations, lenders are required to report data about loan applicants (both those who were approved and those who were not) and regulators essentially look for patterns in the data to determine if there is any wholesale discrimination. The Federal Reserve Bank makes annual reports to congress, these reports include analysis of the reported data - which is probably the closest thing you'll get to data supporting whether or not lenders discriminate based on age.
You also asked,
suppose if one gets the mortgage when he/she is 55 Years or older and has job and sufficient assets ( and is downsizing the home), what is a pros and cons of taking a mortgage from financial perspective ?
The pros and cons don't have any inherent link to your age - all else being the same, they should be the same as the pros and cons at any other age.
Of course, there are some obvious things that probably won't be equal - many people live off retirement savings or other investment income later in life, versus working for a salary. If this means you will be on a fixed income, a mortgage may actually make slightly more sense than other forms of housing, since your payment is fixed for the duration of the loan (versus, say, renting - where the landlord may increase rent to match inflation over time.)
There's also an increased chance that an older person may die before the loan is paid off, compared to someone of a younger age. How this impacts your estate as it's passed on to your heirs may be worth considering, too.
In terms of the mortgage process, lenders typically need proof of income in order to show that you will be able to pay the loan off over time. For someone who is working, this is easily done by providing pay stubs or tax returns. If you are retired, or will retire soon, you may need to work with your lender to make sure you're providing proof of your income and proof that it will be stable over time. Generally speaking, having lots of assets isn't inherently taken as proof of income - especially if the assets are liquid (i.e. cash in a savings account). The lender will want to know that you will have a stable cash flow, whereas just having a pile of money may be more risky since there's nothing stopping you from blowing it all in the first year of the mortgage.