How old are the world's nuclear reactors?

Mean reactor age: 30.3
Median reactor age: 33.0
Mode reactor age: 34.0

import pandas as pd, numpy as np
import matplotlib.pyplot as plt
from matplotlib.patches import Rectangle
import seaborn as sbs

sbs.set(font_scale=1.25)

csv_name = 'reactors.csv'

# Read in the data, discard non-operational reactors, get build year and age.
df = pd.read_csv(csv_name, usecols=[1,5], names=['status', 'dateconnected'],
                 header=None)
df = df[df['status']=='Operational']
df['dateconnected'] = pd.to_datetime(df['dateconnected'])
this_year = pd.datetime.now().year
df['year'] = df['dateconnected'].dt.year
df['age'] = this_year - df['dateconnected'].dt.year

# Count reactors of each age (in years) and sort; find averages.
reactors_by_age = df['age'].value_counts().sort_index()
mean_age = df['age'].mean()
median_age = df['age'].median()
mode_age = df['age'].mode()

print('Mean reactor age: {:.1f}'.format(mean_age))
print('Median reactor age: {:.1f}'.format(median_age))
# NB mode() returned a Pandas Series.
print('Mode reactor age: {:.1f}'.format(mode_age[0]))

# Now convert to build year and plot a bar chart
reactors_by_buildyear = df['year'].value_counts().sort_index()

fig, ax = plt.subplots()
ax.bar(reactors_by_buildyear.index, reactors_by_buildyear,
       fc=sbs.xkcd_rgb["denim blue"])

# Set the y-axis max limit to the largest number greater than the maximum
# number of reactors built in a single year that is divisible by 5.
max_built_per_year = reactors_by_buildyear.max()
ylim = 5 * (max_built_per_year // 5 + 1)
ax.set_ylim(0,ylim)

# Draw some semi-transparent rectangles over significant age ranges.
ages = np.array((30, 40, 50))
opacities = (0.15, 0.25)
for i, age in enumerate(ages[1:]):
    rectangle = Rectangle((this_year-age, 0), age-ages[i-1], ylim,
                    alpha=opacities[i], fc=sbs.xkcd_rgb["denim blue"])
    ax.add_patch(rectangle)

# Labels, twinned axis with reactor ages on ticks.
ax.set_xlabel('Build year')
ax.set_ylabel('Number of reactors')

ax_top = ax.twiny()
ax_top.set_xlim(ax.get_xlim())
ax_top.grid(False)
ax_top.set_xticks((this_year - ages)[::-1])
ax_top.set_xticklabels(['{:2d} years'.format(age) for age in ages[::-1]])
fig.suptitle('Age and build year of currently-operational nuclear reactors')
fig.subplots_adjust(top=0.85)

plt.savefig('reactor_ages.png')
plt.show()