variable dtype does not update when populating a dataframe
I posted a question about this on [StackOverflow](https://stackoverflow.com/questions/54650913/why-doesnt-pandas-update-a-variables-dtype-when-populating-a-dataframe), but though it might be something worth reporting here.

#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd
df = pd.DataFrame(columns = ("Name", "Age"))
df.loc[1] = "Jane", 5
df.loc[2] = "Riley", 24
df.dtypes
```
#### Problem description

In my test:

- with Python 3.5 and Pandas 0.18.1, populating the dataframe does update the "object" dtype of the Age variable to "float64"
- with Python 3.7 and Pandas 0.23.4, populating the dataframe does not update the "object" dtype of the Age variable

Why is that? I couldn't find an explanation in the documentation.

#### Expected Output

The dtype of an ampty variable gets updated when populating it for the first time, similarly to what `infer_objects()` does.

#### Output of ``pd.show_versions()``

<details>
INSTALLED VERSIONS
------------------
commit: None
python: 3.7.1.final.0
python-bits: 64
OS: Linux
OS-release: 4.15.0-45-lowlatency
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_AU.UTF-8
LOCALE: en_AU.UTF-8

pandas: 0.23.4
pytest: 4.0.1
pip: 18.1
setuptools: 40.6.2
Cython: 0.29
numpy: 1.15.4
scipy: 1.1.0
pyarrow: None
xarray: None
IPython: 7.2.0
sphinx: 1.8.2
patsy: 0.5.1
dateutil: 2.7.5
pytz: 2018.7
blosc: None
bottleneck: 1.2.1
tables: 3.4.4
numexpr: 2.6.8
feather: None
matplotlib: 3.0.1
openpyxl: 2.5.11
xlrd: 1.1.0
xlwt: 1.3.0
xlsxwriter: 1.1.2
lxml: 4.2.5
bs4: 4.6.3
html5lib: 1.0.1
sqlalchemy: 1.2.14
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
</details>
