What statistics are there on the age of conversions? By age group, what percentage of converts to Christianity are there after age 70?
By age group, what percentage of converts to evangelical Christianity after 70?
By conversion to Christianity, I take that to mean when a person turns to Christ after repenting before God of their sins.  I do not take that to mean baptism.  The difficulty here is that although Christian denominations (including Evangelical denominations) record baptisms, I am unaware that they record the age of the baptismal candidate.  As for when a person converted to Christianity, I am unaware of any Christian denomination recording statistics on the age of conversion.
In my own experience, although I know the date of my conversion and my age then (47), that statistic has never been recorded anywhere.  The only source of information on the age of conversion would probably come from polls.  I found one conducted by the National Association of Evangelicals (in spring 2015) on the age of conversion:
63% up to age 14
34% 15-29
2% 30 plus
There was no specific information on the sample size of the poll or anything to confirm its authenticity.
Although the American Pew Research Center conducts numerous surveys and studies into Christian statistics/demographics, the one below says absolutely nothing about the age of people when they converted to Christianity.  However, is does mention factors that affect the age of conversion:

Size of youth population. “In 2010, more than a quarter of the world’s total population (27%) was under the age of 15.” Christian youth under 15 were the same as the 27% global average. But an even higher percentage of Muslims (34%) were younger than 15. This higher youth population is one of the reasons that from 2010-2050 Muslims are projected to grow faster than Christians.
Size of old population. In 2010, “11% of the world’s population was at least 60 years old,” 14% of the Christian population was over 60 years old, but only 7% of Muslims were over 60. This is another reason that Muslims are projected to grow faster than Christians.
Switching. A loss of 66 million Christians is projected to come through switching. Most of the loss is projected to come from Christians “joining the ranks of the religiously unaffiliated.”

Religious Self-Identification of the U.S. Adult Population: 1990, 2001, 2008  https://en.wikipedia.org/wiki/Christian_population_growth#Conversion
The World Christian Encyclopedia suggests that approximately 2.7 million people convert to Christianity annually from another religion, with Christianity ranking first in net gains through religious conversion.  However, it does not mention the age of converts.
The older a person becomes, the less likely it is that they would convert from one religion to another.  I doubt there are any statistics on how many people age 70 plus convert.
Edit: Here is the link to the National Association of Evangelicals (thanks to disciple): https://www.nae.net/when-americans-become-christians/